mapi: Delete execmem support code.

No longer used now that we don't dynamically generate dispatch stubs.

Acked-by: Marek Olšák <marek.olsak@amd.com>
Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23451>

meson_options.txt

@@ -460,18 +460,6 @@ option(
                 'with allow_execmem.'
 )
 
-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.'
-)
-
 option(
   'osmesa',
   type : 'boolean',