Allow DROP TABLESPACE to succeed (with a warning) if the pg_tblspc symlink

doesn't exist.  This allows DROP to be used to clean out the pg_tablespace
catalog entry in a situation where a previous DROP attempt failed before
committing but after having removed the directories and symlink.

Per report from William Garrison.  Even though his test case depends on an
unrelated bug in PreventTransactionChain, it's certainly possible for this
situation to arise due to other problems, eg a system crash at just the
right time.