Delete index_members_on_source_id_and_source_type

What does this MR do and why?

!119317 (merged) introduces the index_members_on_source_and_type_and_access_level index which supersedes the existing index_members_on_source_id_and_source_type index. This MR removes the superseded index.

"index_members_on_source_and_type_and_access_level" btree (source_id, source_type, type, access_level)
"index_members_on_source_id_and_source_type" btree (source_id, source_type)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #395545 (closed)