Add the ability to unregister a virtual table module by invoking

sqlite3_create_module() with a NULL sqlite3_module pointer.

FossilOrigin-Name: 31e34fa3390196cdc3178bf120224b08df5ec58fa2c77079ede6e9461a430dad

src/sqlite.h.in

@@ -6624,6 +6624,10 @@ struct sqlite3_index_info {
 ** ^The sqlite3_create_module()
 ** interface is equivalent to sqlite3_create_module_v2() with a NULL
 ** destructor.
+**
+** ^If the third parameter (the pointer to the sqlite3_module object) is
+** NULL then no new module is create and any existing modules with the
+** same name are dropped.
 */
 int sqlite3_create_module(
   sqlite3 *db,               /* SQLite connection to register module with */