Remove heuristic same-transaction test from check_safe_enum_use().

The blacklist mechanism added by the preceding commit directly fixes
most of the practical cases that the same-transaction test was meant
to cover.  What remains is use-cases like

	begin;
	create type e as enum('x');
	alter type e add value 'y';
	-- use 'y' somehow
	commit;

However, because the same-transaction test is heuristic, it fails on
small variants of that, such as renaming the type or changing its
owner.  Rather than try to explain the behavior to users, let's
remove it and just have a rule that the newly added value can't be
used before being committed, full stop.  Perhaps later it will be
worth the implementation effort and overhead to have a more accurate
test for type-was-created-in-this-transaction.  We'll wait for some
field experience with v10 before deciding to do that.

Back-patch to v10.

Discussion: https://postgr.es/m/20170922185904.1448.16585@wrigleys.postgresql.org

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