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>