[RUN AS-IF-FOSS] Remove wrapper methods from Gitlab::Database

What does this MR do?

This module contained various methods that were forwarded to Gitlab::Database::Connection. This commit removes these wrappers in favour of using Gitlab::Database.main instead. This is done to make it explicit what database connection should be used.

See #331776 (closed) and !65262 (merged) for more information.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
  • No: this is a backstage-only change
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.