This reverts commit 136a12ac.

According to belak51 on IRC, this commit broke Allegro, which would no
longer compile.  Applications apparently expect the GLXContextID typedef
to exist in glx.h; removing it breaks them.  A bit of searching around
the internet revealed other complaints since upgrading to Mesa 10.

Cc: "10.0" <mesa-stable@lists.freedesktop.org>