Backport "taglist, plugins: fix compiler warnings with GLib >= 2.76" into 1.22

Fix compiler warnings about not using the return value when freeing the GString segment with g_string_free(.., FALSE):

ignoring return value of ‘g_string_free_and_steal’ declared with attribute ‘warn_unused_result’

which we get with newer GLib versions. These were all harmless.

Part-of: !4273 (merged)