Remove legacy build directory caching in Docker Executor

What does this MR do?

Implements removal of legacy builds directory caching in Docker Executor.

Why was this MR needed?

Fulfilling our previous deprecation notice. Please see #4180 (closed) for details.

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

Closes #4180 (closed)