[TASK] Remove superfluous files/folder/comment created/used by EXT:extension_builder

I guess the roundtrip feature of EXT:extension_builder is not
used to maintain EXT:powermail by the team. Therefore some
files/folder/comment can be removed.

Change-Id: Ia0bf75e9152619e6497f8258f377a597dd2ec413
Resolves: #55125
Releases: 2.1
Reviewed-on: https://review.typo3.org/26916
Reviewed-by: Oliver Klee
Reviewed-by: Alex Kellner
Tested-by: Alex Kellner