.github/workflows: remove 'ci skip' boilerplate

We've never used the "[ci skip]" magic commit header in our history,
across all our repos. This seems to be boilerplate we imported years
ago and have since been copying around our CI configs.

Signed-off-by: David Anderson <danderson@tailscale.com>

.github/workflows/cross-windows.yml

@@ -17,8 +17,6 @@ jobs:
   build:
     runs-on: ubuntu-latest
 
-    if: "!contains(github.event.head_commit.message, '[ci skip]')"
-
     steps:
     - name: Check out code into the Go module directory
       uses: actions/checkout@v3