Remove explanatory comments from includes

I noticed a comment by an include and remembered that I think these
don't really provide much value -- sometimes they are just editorial,
and sometimes they are obsolete.  I think it's better to just remove
them.  Tested by rebuilding.

Approved-By: Andrew Burgess <aburgess@redhat.com>

gdbsupport/common-defs.h

@@ -96,7 +96,7 @@
 #include <stdint.h>
 #include <string.h>
 #ifdef HAVE_STRINGS_H
-#include <strings.h>	/* for strcasecmp and strncasecmp */
+#include <strings.h>
 #endif
 #include <errno.h>
 #if HAVE_ALLOCA_H