Reduce Windows CI to just stable Rust

Author: cuviper

appveyor.yml

@@ -7,37 +7,12 @@ environment:
       CHANNEL: stable
       RUSTFLAGS: --cfg rayon_unstable
 
-    - TARGET: x86_64-pc-windows-gnu
-      CHANNEL: beta
-    - TARGET: x86_64-pc-windows-gnu
-      CHANNEL: beta
-      RUSTFLAGS: --cfg rayon_unstable
-
-    - TARGET: x86_64-pc-windows-gnu
-      CHANNEL: nightly
-    - TARGET: x86_64-pc-windows-gnu
-      CHANNEL: nightly
-      RUSTFLAGS: --cfg rayon_unstable
-
-
     - TARGET: x86_64-pc-windows-msvc
       CHANNEL: stable
     - TARGET: x86_64-pc-windows-msvc
       CHANNEL: stable
       RUSTFLAGS: --cfg rayon_unstable
 
-    - TARGET: x86_64-pc-windows-msvc
-      CHANNEL: beta
-    - TARGET: x86_64-pc-windows-msvc
-      CHANNEL: beta
-      RUSTFLAGS: --cfg rayon_unstable
-
-    - TARGET: x86_64-pc-windows-msvc
-      CHANNEL: nightly
-    - TARGET: x86_64-pc-windows-msvc
-      CHANNEL: nightly
-      RUSTFLAGS: --cfg rayon_unstable
-
 install:
   - curl -sSf -o rustup-init.exe https://win.rustup.rs
   - rustup-init.exe --default-host %TARGET% --default-toolchain %CHANNEL% -y