flac2mp3: tests: replace/remove some comments

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
flac2mp3: remove some FIXMEs

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
TagUtils: add a comment explaining why some functionality is present

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>