tgsi: Drop uses_fogcoord and uses_frontfacing from tgsi_scan.

tgsi: Drop uses_fogcoord and uses_frontfacing from tgsi_scan.

It was not used anywhere; the code was buggy (it didn't take care of
indirect registers and could potential cause buffer underflows) and the
same effect can now be easily achieved by just by looking at
input_semantic_name[] and input_usage_mask[].