Remove option USER_LDT and everything depending on it.

Remove machdep.userldt sysctl.
Remove i386_[gs]et_ldt syscall stub from libi386.
Remove i386_[gs]et_ldt regression test.
ok mlarkin@ millert@ guenther@

src/etc/etc.i386/sysctl.conf

@@ -2,5 +2,3 @@
 #machdep.apmhalt=1		# 1=powerdown hack, try if halt -p doesn't work
 #machdep.kbdreset=1		# permit console CTRL-ALT-DEL to do a nice halt
 #machdep.lidsuspend=0		# do not suspend laptop upon lid closing
-#machdep.userldt=1		# allow userland programs to play with ldt,
-				# required by some ports