st/mesa: don't force per-sample interp if only sampleid/pos are used

The OES extensions clarify this behaviour to differentiate between
per-sample invocation and per-sample interpolation. Using sampleid/pos
will force per-sample invocation but not per-sample interpolation.

See https://www.khronos.org/bugzilla/show_bug.cgi?id=1462

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/state_tracker/st_program.c

@@ -573,10 +573,6 @@ st_translate_fragment_program(struct st_context *st,
          else
             interpLocation[slot] = TGSI_INTERPOLATE_LOC_CENTER;
 
-         if (stfp->Base.Base.SystemValuesRead & (SYSTEM_BIT_SAMPLE_ID |
-                                                 SYSTEM_BIT_SAMPLE_POS))
-            interpLocation[slot] = TGSI_INTERPOLATE_LOC_SAMPLE;
-
          switch (attr) {
          case VARYING_SLOT_POS:
             input_semantic_name[slot] = TGSI_SEMANTIC_POSITION;