[TASK] Remove not maintained md5 in ext_emconf

The serialized array _md5_values_when_last_written is not
maintained for core extensions since ages and just contains
a long string with outdated information.
It is set to an empty string now.

Change-Id: Iae1de150dd0da4ef9ccd993bdc0e75e5eb4c62c1
Resolves: #46699
Releases: 6.1
Reviewed-on: https://review.typo3.org/19325
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn