gmt inset shall not accept -X -Y

While the -X and -Y options serve no purpose in the gmt inset call, and are not listed as allowable options neither in synopsis nor documentation, we did in fact not prohibit them, leading to troubles (#6288).
This PR slams that door shot.  No example or tests used -X -Y since that was never the plan.

src/inset.c

@@ -36,7 +36,7 @@
 #define THIS_MODULE_PURPOSE "Manage figure inset setup and completion"
 #define THIS_MODULE_KEYS ">X}"
 #define THIS_MODULE_NEEDS "JR"
-#define THIS_MODULE_OPTIONS "JRVXY"
+#define THIS_MODULE_OPTIONS "JRV"
 
 /* Control structure for inset */