lastgenre: remove unnecessary check
lastgenre: remove unnecessary check

In _resolv_genres wo do not have to explicitly check if self.c14n_branches is
not empty. The tree will have been loaded when self.canonicalize is truthy. Also
if self.c14n_branches would be empty the canonicalization is a no-op anyway...

beetbox/beets,ibmibmibm/beets,shamangeorge/beets,beetbox/beets,SusannaMaria/beets,sampsyo/beets,ibmibmibm/beets,SusannaMaria/beets,sampsyo/beets,ibmibmibm/beets,jackwilsdon/beets,jackwilsdon/beets,shamangeorge/beets,sampsyo/beets,beetbox/beets,jackwilsdon/beets,shamangeorge/beets,SusannaMaria/beets,jackwilsdon/beets,sampsyo/beets,ibmibmibm/beets,SusannaMaria/beets,beetbox/beets,shamangeorge/beets