Bug 1804178 - [3/6] Remove explicit sorting of TOML output r=glandium

Remove explicit sorting, as this is no longer needed to guarantee
determinism as of Python 3.6 (our minimum-supported version).

(This is a separate commit solely to make it clear that the reordering
of `.cargo/config.in` is indeed merely a reordering and contains no
hidden functional changes.)

Differential Revision: https://phabricator.services.mozilla.com/D164152