Remove qt5 nightly jobs

They require maintenance, but we don't have a great need for Qt5 builds.
See https://github.com/qutebrowser/qutebrowser/issues/8260

All the Qt5 switches are still in tox, the build release script and the
nsis installer.

.github/workflows/nightly.yml

@@ -14,20 +14,6 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: macos-12
-            toxenv: build-release-qt5
-            name: qt5-macos
-          - os: windows-2019
-            toxenv: build-release-qt5
-            name: qt5-windows
-          - os: macos-12
-            args: --debug
-            toxenv: build-release-qt5
-            name: qt5-macos-debug
-          - os: windows-2019
-            args: --debug
-            toxenv: build-release-qt5
-            name: qt5-windows-debug
           - os: macos-12
             toxenv: build-release
             name: macos-intel