cmake: Remove needless checks for Taglib format support.

The recent commit 9a54d6e7 to use
FindTaglib from ECM breaks detection of Ogg Opus support in Taglib for
some reason. This causes crashes when playing Ogg Opus files because
their relevant Taglib::File subclass is not created from our MediaFiles
factory.

However the check is needless. Taglib support for ASF and MP4 files has
been standard since 2012 or so, and Ogg Opus support was made standard
around the same time frame. So rather than check for these as optional
features, just assume that supported versions of Taglib support these
formats.