glsl: Drop the dead MOD_TO_FLOOR path.

It's now called lower_fmod in NIR.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8044>

src/intel/compiler/brw_vec4_nir.cpp

@@ -1900,7 +1900,7 @@ vec4_visitor::nir_emit_alu(nir_alu_instr *instr)
       unreachable("not reached: should be lowered by DIV_TO_MUL_RCP in the compiler");
 
    case nir_op_fmod:
-      unreachable("not reached: should be lowered by MOD_TO_FLOOR in the compiler");
+      unreachable("not reached: should be lowered by lower_fmod in the compiler");
 
    case nir_op_fsub:
    case nir_op_isub: