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

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].

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