[TASK] Remove _md5_values_when_last_written from ext_emconf

The md5 hashes of files are unmaintained for core extensions
and do not have a real (security) benefit. They are also unused
in new exnensionmanager and only pollute ext_emconf files.

A good solution to check the status file of extensions
should work with public key methods, the current solution gives
no benefit, so the content of ext_emconf is now cleaned up by
removing those hashes.