Remove no longer used appveyor (#1401)

.appveyor.yml

@@ -1,26 +0,0 @@
-install:
-  - C:\Python36\python.exe -m pip install mypy
-  - C:\Python36\python.exe -m pip install -e .[d]
-
-# Not a C# project
-build: off
-
-test_script:
-  - C:\Python36\python.exe tests/test_black.py
-  - C:\Python36\python.exe -m mypy src/black src/blackd tests/test_black.py src/blib2to3
-
-after_test:
-  - C:\Python36\python.exe -m pip install pyinstaller
-  - "%CMD_IN_ENV% C:\\Python36\\python.exe -m PyInstaller --clean -F --add-data
-    src/blib2to3/;blib2to3 src/black/__init__.py"
-
-artifacts:
-  - path: dist/black.exe
-
-deploy:
-  provider: GitHub
-  description: ""
-  auth_token:
-    secure: uplI9CJ2dTGcEBCbZuIn+Qb4rC38hOoRSH9lcwuGCr5g9fSnhK1MZdNT6Cjf/mFL
-  on:
-    APPVEYOR_REPO_TAG: true