doc: remove imprecise and redundant testing text

Remove "All commits...are tested on these platforms." for the following reasons:

• It's somewhat redundant. The surrounding text makes it clear that we test on these platforms.
• It's imprecise. Commits, such as those that only affect documentation or linting do not get tested on all the platforms. (Well, perhaps they do in an after-the-fact kind of way when a subsequent commit gets tested, but I think that's a stretch.)

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines