Remove the check about whether MALLOC_EXTRA_SANITY is defined,

surrounding the undef'ing it.  It does not seem necessary to
undef some symbol that is not exist, and gcc does not complain
about whether a symbol is exist before #undef'ing it out.

Spotted by:	mingyanguo via ChinaUnix.net forum
Reviewed by:	phk

lib/libc/stdlib/malloc.c

@@ -18,9 +18,7 @@ __FBSDID("$FreeBSD$");
  * any good unless you fiddle with the internals of malloc or want
  * to catch random pointer corruption as early as possible.
  */
-#ifndef MALLOC_EXTRA_SANITY
 #undef MALLOC_EXTRA_SANITY
-#endif
 
 /*
  * What to use for Junk.  This is the byte value we use to fill with