gdb: remove unused include in symtab.h

This include is unused, according to clangd.

Change-Id: Ifbc2fe75b02c9ae9b3e2f1184bbcc4dc7095a554

gdb/symtab.h

diff --git a/gdb/symtab.h b/gdb/symtab.h
index e6b2882..cd9220c 100644
--- a/gdb/symtab.h
+++ b/gdb/symtab.h
@@ -33,7 +33,6 @@
 #include <optional>
 #include <string_view>
 #include "gdbsupport/next-iterator.h"
-#include "gdbsupport/iterator-range.h"
 #include "completer.h"
 #include "gdb-demangle.h"
 #include "split-name.h"