tests: drop here-doc check from internal chain-linter

tests: drop here-doc check from internal chain-linter

Commit 99a64e4b73c (tests: lint for run-away here-doc, 2017-03-22)
tweaked the chain-lint test to catch unclosed here-docs. It works by
adding an extra "echo" command after the test snippet, and checking that
it is run (if it gets swallowed by a here-doc, naturally it is not run).

The downside here is that we introduced an extra $() substitution, which
happens in a subshell. This has a measurable performance impact when
run for many tests.

The tradeoff in safety was undoubtedly worth it when 99a64e4b73c was
written. But since the external chainlint.pl learned to find these
recently, we can just rely on it. By switching back to a simpler
chain-lint, hyperfine reports a measurable speedup on t3070 (which has
1800 tests):

  'HEAD' ran
    1.12 ± 0.01 times faster than 'HEAD~1'

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>