When deleting an SQL function that does not exist, return without doing

anything at all rather than creating a tombstone function.  In this way,
function deletes that happen inside virtual-table destructors that are run
when a database connection is closing do not create new tombstones in the
function table after the function table has already been purged.
[forum:/forumpost/726219164b|forum post 726219164b].

FossilOrigin-Name: 391c73132c80df944fb49a17d8fe78203c54ac48f968ee9dd9dd8c769c0b4b10

test/vtab1.test

@@ -983,6 +983,7 @@ do_test vtab1.10-5 {
 proc match_func {args} {return ""}
 do_test vtab1.10-6 {
   set echo_module ""
+  sqlite_delete_function db match
   db function match match_func
   execsql {
     SELECT * FROM e WHERE match('pattern', rowid, 'pattern2');