mp3_afh: Always create id3v2 tags.

When para_afh is asked to modify the tags of an mp3 file, it looks
at both version 1 and version 2 id3 tags. Currently, if the file
contains only id3v1 tags, para_afh will replace the tag contents but
will not create a version 2 tag. This is unfortunate if the length of
a given tag value exceeds the fixed-size byte limit of id3v1 because
it results in a truncated value and there is no way to specify that
an id3v2 tag should be created (which could store tags of any size).

This commit makes para_afh always write id3v2 tags. This actually
simplifies the code a bit because with this change the v2_tag variable
can never be NULL, allowing to get rid of a conditional.