CI: Remove unneeded install of git in maint-checks
This was added in 9357a8fd "ci: add shebang to the GitLab CI" as
part of !990 (merged) to the maint-checks
job; but the actual additional
check was added to the doc-features
job (by mistake, fixed in
!1490 (merged)); and, that shebang check script doesn't need git anyway.
Found while reviewing !1490 (merged), CC @emilengler