UI,libobs,obs-outputs: Remove HAVE_OBSCONFIG_H ifdefs
With the removal of all legacy code paths, obsconfig.h always exists and
the compile definition always gets set. As such, it's no longer
necessary to check for it.
As removing the definition itself could be seen as a breaking change,
this simply moves the definition to pc.in and cmake.in files for now to
preserve the value for plugins that might expect this to be set. We may
remove the definition entirely in a later release.