Remove some unused includes

I noticed a few spots that include gnu-stabs.h but that do not need
to.  This patch removes these unnecessary includes.  Tested by
rebuilding.

gdb/objfiles.c

@@ -27,7 +27,6 @@
 #include "symtab.h"
 #include "symfile.h"
 #include "objfiles.h"
-#include "gdb-stabs.h"
 #include "target.h"
 #include "bcache.h"
 #include "expression.h"

gdb/remote.c

@@ -30,7 +30,6 @@
 #include "process-stratum-target.h"
 #include "gdbcmd.h"
 #include "objfiles.h"
-#include "gdb-stabs.h"
 #include "gdbthread.h"
 #include "remote.h"
 #include "remote-notif.h"

gdb/symfile.c

@@ -39,7 +39,6 @@
 #include "inferior.h"
 #include "regcache.h"
 #include "filenames.h"		/* for DOSish file names */
-#include "gdb-stabs.h"
 #include "gdbsupport/gdb_obstack.h"
 #include "completer.h"
 #include "bcache.h"