i965/fs: Remove logic to keep track of MRF metadata in lower_load_payload().
i965/fs: Remove logic to keep track of MRF metadata in lower_load_payload().

MRFs cannot be read from anyway so they cannot possibly be a valid
source of LOAD_PAYLOAD.

Reviewed-by: Jason Ekstrand <b186559eaedf698788888c8a2559b4efb4db10d6@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler