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.)

PR-URL: #30763
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

Author: Trott

BUILDING.md

@@ -69,13 +69,11 @@ There are three support tiers:
 
 * **Tier 1**: These platforms represent the majority of Node.js users. The
   Node.js Build Working Group maintains infrastructure for full test coverage.
-  All commits to the Node.js repository are tested on these platforms. Test
-  failures on tier 1 platforms will block releases.
+  Test failures on tier 1 platforms will block releases.
 * **Tier 2**: These platforms represent smaller segments of the Node.js user
   base. The Node.js Build Working Group maintains infrastructure for full test
-  coverage. All commits to the Node.js repository are tested on these platforms.
-  Test failures on tier 2 platforms will block releases. Delays in release of
-  binaries for these platforms are acceptable where necessary due to
+  coverage. Test failures on tier 2 platforms will block releases. Delays in
+  release of binaries for these platforms are acceptable where necessary due to
   infrastructure concerns.
 * **Experimental**: May not compile or test suite may not pass. The core team
   does not create releases for these platforms. Test failures on experimental