bootstrap: exclude link_jobs from check_ci_llvm! checks

This option is largely there to help people to manage the memory usage
on their systems during the LLVM build. The linking phase is as usual
are the heaviest part of the build and if in an unlucky conincidence the
circumstances align to kick off N_CORES links at the same time, not even
hundreds of GiB of memory may suffice. It makes a lot of sense for
developers to set&forget this option unconditionally.

Not to mention, this option does not, in any way, affect the generated
code (at least as far as I know.) It really doesn’t matter what
option the CI build LLVM used here and/or if it matches with the user’s
configuration.

Finally, 0 actual link jobs implied by `download-ci-llvm` is guaranteed
to stay within the limits that are reasonable to set with this option.

Author: nagisa

src/bootstrap/src/core/config/config.rs

@@ -1776,7 +1776,6 @@ impl Config {
                 check_ci_llvm!(static_libstdcpp);
                 check_ci_llvm!(targets);
                 check_ci_llvm!(experimental_targets);
-                check_ci_llvm!(link_jobs);
                 check_ci_llvm!(clang_cl);
                 check_ci_llvm!(version_suffix);
                 check_ci_llvm!(cflags);