drop -uninstalled.pc files

These were meant to provide the ability to use GLib directly out of its
build tree but have a couple of rather serious limitations that prevent
their serious use:

 - these files only worked with programs built using libtool

 - for non-trivial uses, other environment variables will need to be
   setup to allow GLib to function properly

 - the system doesn't stack well when attempting to use multiple
   libraries all uninstalled

The jhbuild --prefix= style system works much better in every way, so we
remove this old hack.