t4216: fix no-op test that breaks TAP output

t4216: fix no-op test that breaks TAP output

In t4216 we have have a prerequisite that is active in case the system's
`char` type is signed by default. This prerequisite isn't really used by
anything though: while it is used to guard one of our tests, that
specific test is essentially a no-op. So all this infrastructure does is
to provide some debugging hint to a reader that pays a lot of attention.

Besides that, the way we set up the prerequisite also results in broken
TAP output on systems where `char` is unsigned by default: we use
`test_cmp()` to diff two files outside of of any test body, and if the
files differ we enable the prerequisite. If so, the call to `test_cmp()`
would also print output, and that output is of course not valid TAP
output.

That wasn't a problem before 389c83025d (t: let prove fail when parsing
invalid TAP output, 2026-06-04), because our TAP parser was configured
to be lenient. But starting with that commit, t4216 is now failing on
systems with unsigned chars.

Drop the whole infrastructure. The prerequisite is not used anywhere
else, and the only location where it's used doesn't really provide much
value.

Reported-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Tested-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>