drop clangarm64-build (#5322)

This is no longer needed, as Git for Windows' regular release process
(as well as regular CI builds) now support the clang-aarch64 parts out
of the box.

I noticed that this is still part of Git for Windows' branch thicket
when reviewing https://github.com/microsoft/git/pull/713

.github/workflows/clangarm64-build.yml

@@ -1,25 +0,0 @@
-name: CLANG build ARM64
-
-on:
-  workflow_dispatch:
-
-defaults:
-  run:
-    shell: bash
-
-jobs:
-  clang-build:
-    runs-on: [Windows, ARM64]
-    env:
-      NO_PERL: 1
-    steps:
-      - uses: actions/checkout@v4
-      - uses: git-for-windows/setup-git-for-windows-sdk@v1
-        with:
-          flavor: makepkg-git
-          architecture: aarch64
-          # This assumes that the job is running on a self-hosted runner,
-          # in which case we need to cleanup SDK files.
-          cleanup: true
-      - name: Build Git CLANGARM64
-        run: make -j`nproc`