Remove unused `index_environments_for_name_search_within_folder`

What does this MR do and why?

This index was used for a query that was changed in Fix search within Environments folder (!146154 - merged)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Migration output

$ bundle exec rails db:migrate
main: == [advisory_lock_connection] object_id: 119400, pg_backend_pid: 53732
main: == 20240306030118 DropIndexEnvironmentsForNameSearchWithinFolder: migrating ===
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0163s
main: -- indexes(:environments)
main:    -> 0.0046s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- remove_index(:environments, {:algorithm=>:concurrently, :name=>"index_environments_for_name_search_within_folder"})
main:    -> 0.0037s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: == 20240306030118 DropIndexEnvironmentsForNameSearchWithinFolder: migrated (0.0385s) 

main: == [advisory_lock_connection] object_id: 119400, pg_backend_pid: 53732

How to set up and validate locally

N/A

Related to #427537 (closed)