gdb: remove unused include from gdbtypes.c

clangd says it's not used.

Change-Id: I49ae57264599b490c3fe4c61c05a11e7c0891056
diff --git a/gdb/gdbtypes.c b/gdb/gdbtypes.c
index 6736de9..975e124 100644
--- a/gdb/gdbtypes.c
+++ b/gdb/gdbtypes.c
@@ -32,7 +32,6 @@
 #include "complaints.h"
 #include "cli/cli-cmds.h"
 #include "cp-abi.h"
-#include "hashtab.h"
 #include "cp-support.h"
 #include "bcache.h"
 #include "dwarf2/loc.h"