[BUGFIX] Improve l10n_mode config for sys_file_reference

Since the rendering in the Frontend does not merge the values
the mergeIfNotBlank setting is replaced with prefixLangTitle for
the alternative, description and title field.

For the link field the mergeIfNotBlank setting is removed because
a prefix does not make sense there.

Releases: 6.3, 6.2
Resolves: #59972
Change-Id: I242b687a7ce851270022b86561f13eacea4d3949
Reviewed-on: http://review.typo3.org/31212
Reviewed-by: Marcin Sągol <marcin@soee.pl>
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>