test: drop the implicit prereq check mechanism from test_expect_*

test: drop the implicit prereq check mechanism from test_expect_*

The only place where we use the implicit prereq check is T000-basic.sh
where we check that it works. It's an added complication that we don't
use. Remove it.

The test_have_prereq function can still be used for the same effect in
subtests that use test_begin_subtest. For now, this will make it
impossible to have prereqs in one-line subtests that don't require
test_begin_subtest. This will be fixed in follow-up work.