Remove unused index on ci_pipeline_metadata

What does this MR do and why?

Remove unused index on ci_pipeline_metadata.

Since an index on [name, pipeline_id] has been introduced (in !140607 (merged)), we can remove the old index on [pipeline_id, name].

The new index has already been successfully created on gitlab.com. Verified on database-lab: https://console.postgres.ai/gitlab/gitlab-production-ci/sessions/25293/commands/80515.

pipeline_id is the table's primary key, so there is already an index on it. See all indexes of the table: https://console.postgres.ai/gitlab/gitlab-production-ci/sessions/25293/commands/80523.

main: == [advisory_lock_connection] object_id: 183000, pg_backend_pid: 2210
main: == 20240212120012 RemoveIndexOnPipelineMetadata: migrating ====================
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.1414s
main: -- indexes(:ci_pipeline_metadata)
main:    -> 0.0100s
main: -- remove_index(:ci_pipeline_metadata, {:algorithm=>:concurrently, :name=>"index_pipeline_metadata_on_pipeline_id_name_text_pattern"})
main:    -> 0.0069s
main: == 20240212120012 RemoveIndexOnPipelineMetadata: migrated (0.2011s) ===========

main: == [advisory_lock_connection] object_id: 183000, pg_backend_pid: 2210
ci: == [advisory_lock_connection] object_id: 183220, pg_backend_pid: 2212
ci: == 20240212120012 RemoveIndexOnPipelineMetadata: migrating ====================
ci: -- transaction_open?(nil)
ci:    -> 0.0000s
ci: -- view_exists?(:postgres_partitions)
ci:    -> 0.0011s
ci: -- indexes(:ci_pipeline_metadata)
ci:    -> 0.0086s
ci: -- remove_index(:ci_pipeline_metadata, {:algorithm=>:concurrently, :name=>"index_pipeline_metadata_on_pipeline_id_name_text_pattern"})
ci:    -> 0.0049s
ci: == 20240212120012 RemoveIndexOnPipelineMetadata: migrated (0.0409s) ===========

Down migration:

main: == [advisory_lock_connection] object_id: 182700, pg_backend_pid: 6208
main: == 20240212120012 RemoveIndexOnPipelineMetadata: reverting ====================
main: -- index_name_exists?(:ci_pipeline_metadata, "index_pipeline_metadata_on_pipeline_id_name_text_pattern")
main:    -> 0.0451s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0012s
main: -- index_exists?(:ci_pipeline_metadata, "pipeline_id, name text_pattern_ops", {:name=>"index_pipeline_metadata_on_pipeline_id_name_text_pattern", :algorithm=>:concurrently})
main:    -> 0.0053s
main: -- add_index(:ci_pipeline_metadata, "pipeline_id, name text_pattern_ops", {:name=>"index_pipeline_metadata_on_pipeline_id_name_text_pattern", :algorithm=>:concurrently})
main:    -> 0.0072s
main: == 20240212120012 RemoveIndexOnPipelineMetadata: reverted (0.1100s) ===========

ci: == [advisory_lock_connection] object_id: 182640, pg_backend_pid: 6226
ci: == 20240212120012 RemoveIndexOnPipelineMetadata: reverting ====================
ci: -- index_name_exists?(:ci_pipeline_metadata, "index_pipeline_metadata_on_pipeline_id_name_text_pattern")
ci:    -> 0.0340s
ci: -- transaction_open?(nil)
ci:    -> 0.0000s
ci: -- view_exists?(:postgres_partitions)
ci:    -> 0.0011s
ci: -- index_exists?(:ci_pipeline_metadata, "pipeline_id, name text_pattern_ops", {:name=>"index_pipeline_metadata_on_pipeline_id_name_text_pattern", :algorithm=>:concurrently})
ci:    -> 0.0068s
ci: -- add_index(:ci_pipeline_metadata, "pipeline_id, name text_pattern_ops", {:name=>"index_pipeline_metadata_on_pipeline_id_name_text_pattern", :algorithm=>:concurrently})
ci:    -> 0.0089s
ci: == 20240212120012 RemoveIndexOnPipelineMetadata: reverted (0.0817s) ===========

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