Simplify linear searches in FilterConfigCache

No change in functionality, just eliminated a redundant comparison like
FilterConfigCache::GetImportFormatNumberForExtension already does.

Change-Id: I7b04eae2ae7e79f0804ad958d61771758545bf0a
Reviewed-on: https://gerrit.libreoffice.org/8502
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>