libc: remove reference to non-existent readme

remove reference to non-existent readme in libc.

Pointing to the documentation page doesn't make sense in this case,
because it doesn't explain the use of `#undef XXX` for this case anyway.

Signed-off-by: raiden00pl <raiden00@railab.me>

libs/libc/stdlib/lib_aligned_alloc.c

@@ -32,7 +32,7 @@
  * Public Functions
  ****************************************************************************/
 
-#undef aligned_alloc /* See mm/README.txt */
+#undef aligned_alloc
 FAR void *aligned_alloc(size_t align, size_t size)
 {
   return lib_memalign(align, size);