doc: remove "multiple variants" from BUILDING.md

The statement that tests for tier 1 platforms are run on multiple
variants is not true. We usually only run on one variant of macOS.
Remove "multiple", which simplifies and clarifies the statements anyway.

Author: Trott

BUILDING.md

@@ -69,14 +69,14 @@ 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 multiple variants of these
-  platforms. Test failures on tier 1 platforms will block releases.
+  All commits to the Node.js repository are tested on these platforms. 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 multiple
-  variants of these platforms where practical. 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.
+  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
+  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
   platforms do not block releases. Contributions to improve support for these