Drop translatable pspec nick/blurbs from properties in GIO

The same reasoning as in gtk#4904 (closed) applies to the properties in GIO: the translatable nick/blurb are not used by any tools, and even if they were that would result in a rubbish user experience. It’s better for tools to point to the documentation rather than consuming nicks/blurbs from properties (apart from in the special case of GStreamer).

We could drop the translation markers (P_()) from properties, but the untranslated strings would still be useless and (slightly) bloat the library size. So just drop the strings entirely and replace them with NULL.

!3407 (merged) gives an example of the changes required.