implement removal of metadata fields

not working because the writer from KFileMetaData is not able to do that
2 jobs for !178 with modifyMetadata in 11 minutes and 13 seconds (queued for 106 minutes and 7 seconds)
detached
Status Job ID Name Coverage
  Build
passed #74505
linux kf5-qt5 qt5.12

00:10:51

85.0%
passed #74504
reuse

00:00:22