mesa: Remove dynamic dispatch stubs
While reviewing ChromeOS's build vs what we test in CI, I noticed that we have this option disabled downstream:
option(
'execmem',
type : 'boolean',
value : true,
description : 'Enable execmem support. Without execmem, glapi will fail ' +
'to generate dynamic glapi stubs when entrypoints unknown ' +
'to glapi but known to DRI drivers are requested in ' +
'eglGetProcAddress or glXGetProcAddress. This should be ' +
'enabled unless the platform can guarantee glapi and DRI ' +
'drivers are always built from the same source tree.'
Given that Mesa no longer supports loading DRI drivers not matched to the Mesa build, we should be able to delete this glapi != DRI driver support code.