Bug 1641790 - Move --with-system-nss to python configure. r=firefox-build-system-reviewers,rstewart

Version of NSS >= 3.27 have a pkg-config file. We're now requiring 3.53,
so while moving, just use pkg-config, which is simpler.

The old-configure check that rejected some untested platforms for
in-tree NSS is actually rejecting none: the accepted platforms cover all
the supported ones, so we remove that check.

And because building with system NSS without system NSPR doesn't make
sense, imply the latter when the former is used.

Differential Revision: https://phabricator.services.mozilla.com/D77428