Remove old "check for P4" in sha256-586.pl

Historically, OPENSSL_ia32cap_P used reserved bit 20 to trigger some
variation in the RC4 implementation. That same bit was used in
upstream's f889bb03841c69330d1e14a873c9982d3702f2d0 to disable some
optimization on the Pentium 4.

We've long since always cleared that flag (see cpu_intel.c), so this is
dead code. Remove it.

Change-Id: Ib9c0d88235617941833eb8aabb17a4713bdf6606
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/65873
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Bob Beck <bbe@google.com>

crypto/fipsmodule/sha/asm/sha256-586.pl

@@ -211,8 +211,6 @@ sub BODY_00_15() {
 	&picmeup("edx","OPENSSL_ia32cap_P",$K256,&label("K256"));
 	&mov	("ecx",&DWP(0,"edx"));
 	&mov	("ebx",&DWP(4,"edx"));
-	&test	("ecx",1<<20);		# check for P4
-	&jnz	(&label("loop"));
 	&mov	("edx",&DWP(8,"edx"))	if ($xmm);
 	&test	("ecx",1<<24);		# check for FXSR
 	&jz	($unroll_after?&label("no_xmm"):&label("loop"));