Remove GL "DispatchOverride" code

[Ian Romanick]

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Are there any objections to removing the last remnants of Mesa's
"dispatch override" code?  There are a few bits left in Mesa, some
function pointers in glcore.h, and some no-op stubs in the server-side
GLX code.  None of the code is *ever* executed (i.e., there are no calls
to any of it in either source tree).

Since this is all dead code, I'd like to gut it all out.
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