Remove dead code in i386 cpu_throw().

Curpmap must be already valid when cpu_throw() is called, even for early
AP startup.

Suggested by:	alc
Reviewed by:	alc, markj
Sponsored by:	The FreeBSD Foundation
Approved by:	re (marius)
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D16893

sys/i386/i386/swtch.s

@@ -74,16 +74,12 @@
  */
 ENTRY(cpu_throw)
 	movl	PCPU(CPUID), %esi
-	movl	4(%esp),%ecx			/* Old thread */
-	testl	%ecx,%ecx			/* no thread? */
-	jz	1f
 	/* release bit from old pm_active */
 	movl	PCPU(CURPMAP), %ebx
 #ifdef SMP
 	lock
 #endif
 	btrl	%esi, PM_ACTIVE(%ebx)		/* clear old */
-1:
 	movl	8(%esp),%ecx			/* New thread */
 	movl	TD_PCB(%ecx),%edx
 	/* set bit in new pm_active */