Remove transaction memory metrics

What does this MR do?

Remove transaction memory metrics.

This removes the gitlab_transaction_allocated_memory_bytes histogram.

This was measuring the memory before and after a transaction. Which made sense in a single threaded environment, but it doesn't in a multithreaded one.

It also had high cardinality.

Part of gitlab-com/gl-infra/scalability#460 (closed)

This metric is currently not used in the runbooks repo for alerts or dashboards.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team