Historically, client.mk was not invoked with -jn because it would create
race conditions, but that was actually mostly solved by the addition of
`.NOTPARALLEL` in bug 422986, although the mechanism of adding -jn via
`MOZ_MAKE_FLAGS` or `MOZ_PARALLEL_BUILD` has continued well past that.

Nowadays, client.mk is only invoked by mach (it will even bail out if
that's not the case) and only has one target (`build`) and no
dependencies.

This means we don't need to rely on `MOZ_PARALLEL_BUILD` to pass `-jn` in
some cases, and can just always invoke `make -f client.mk` with `-jn`, even
when we just want no parallelism, in which case we can use `-j1`.

This, in turn, allows to remove the extra allow_parallel argument to
`_run_make`, and only rely on `num_jobs`, and to remove some of the
multiple ways the `n` in `-jn` could be set.

Differential Revision: https://phabricator.services.mozilla.com/D124729