x86inc: Drop tzcnt workaround

It is no longer needed now that we've bumped the version requirement of yasm to 1.2.0.

common/x86/x86inc.asm

@@ -1358,10 +1358,6 @@ FMA_INSTR  pmacsdd,  pmulld, paddd
 FMA_INSTR  pmacsww,  pmullw, paddw
 FMA_INSTR pmadcswd, pmaddwd, paddd
 
-; tzcnt is equivalent to "rep bsf" and is backwards-compatible with bsf.
-; This lets us use tzcnt without bumping the yasm version requirement yet.
-%define tzcnt rep bsf
-
 ; convert FMA4 to FMA3 if possible
 %macro FMA4_INSTR 4
     %macro %1 4-8 %1, %2, %3, %4