QA: run ubuntu-only (drop os matrix from the QA group)

Aqua/JET are platform-independent, so the windows/macos QA jobs were
wasted CI. Remove the os axis from the [QA] group in test/test_groups.toml
so QA runs only on the default ubuntu-latest. Core/functional groups keep
their multi-OS coverage.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas

test/test_groups.toml

@@ -4,7 +4,6 @@ os = ["ubuntu-latest", "macos-latest", "windows-latest"]
 
 [QA]
 versions = ["1", "lts", "pre"]
-os = ["ubuntu-latest", "macos-latest", "windows-latest"]
 
 [NoPre]
 versions = ["1", "lts", "pre"]