[BUGFIX][CONF] Remove unused EM related settings

The settings em_wsdlURL and em_mirrorListURL are not used anymore in
the Extension Manager and should be removed.

Change-Id: I70003ac547cb0cc9bfcf47bc90e174e280b30052
Resolves: #48179
Branches: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/20807
Reviewed-by: Wouter Wolters
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe