...left over from 26b40fcf "Moving mysqlc into
connectivity as a library".  (Apparently, for one, a module's Files that don't
exist are ignored, so 8ecf5e18 "Build fix, make
install: mysql-connector-ooo extension is gone" removing
gid_File_Oxt_MySQLConnector but not
gid_Module_Optional_Extensions_MySQLConnector referencing it didn't cause
trouble; and for another, an empty module is ignored, so there were no
extension-mysql-connector packages generated any more---but better clean up the
junk anyway.)

Change-Id: If598a968dfbbe9b5f16d735e8011e192cbd4178b
Reviewed-on: https://gerrit.libreoffice.org/59669
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>