Remove unused enums from Pidgin

None of the listed headers define any public & used enums, so they can be removed from the Meson list.

But actually, none of the headers in Pidgin define any enums, so remove the whole `pidginenums.h`. We can re-add it later if needed.

Testing Done:
Compiled only.

Reviewed at https://reviews.imfreedom.org/r/2208/