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>

gdb/eval.c

@@ -25,7 +25,7 @@
 #include "target.h"
 #include "frame.h"
 #include "gdbthread.h"
-#include "language.h"		/* For CAST_IS_CONVERSION.  */
+#include "language.h"
 #include "cp-abi.h"
 #include "infcall.h"
 #include "objc-lang.h"