ci: replace git-validation with a small shell script

Since commit def70012b9 git-validation is only used to check if the
commit subject is less than 90 characters.

Drop the vendored git-validation Go tool and the .gitvalidation make
target in favor of hack/commit-subject-check.sh.

This removes a Go build dependency and a vendored tree from
test/tools/ while keeping the same CI and local behavior.

Note the now-removed GIT_CHECK_EXCLUDE was not used by gitvalidation
since commit def70012b9 because it was not checking any specific
files, just the commit subject lengths.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>