Remove AppVeyor pipeline (#2949)

Work is already covered by tests in CI PR. Will soon be added to CI Push as well.

.appveyor.yml

@@ -1,42 +0,0 @@
-version: 1.0.{build}
-
-clone_depth: 20
-
-cache:
-  - C:\Users\appveyor\AppData\Roaming\Racket
-
-build_script:
-- '"c:\Program Files (x86)\Microsoft Visual Studio 12.0\vc\vcvarsall.bat" x86'
-- echo %cd%
-- nmake win32-in-place PKGS="racket-test unstable-flonum-lib net-test"
-
-test_script:
-- echo %cd%
-- racket\raco.exe pkg show -l
-- racket\racket.exe -l tests/racket/test
-- racket\racket.exe -l tests/racket/contract/all
-- racket\raco.exe test -l tests/json/json
-- racket\raco.exe test -l tests/file/main
-- racket\raco.exe test -l tests/net/head
-- racket\raco.exe test -l tests/net/uri-codec
-- racket\raco.exe test -l tests/net/url
-- racket\raco.exe test -l tests/net/url-port
-- racket\raco.exe test -l tests/net/encoders
-- racket\raco.exe test -l tests/openssl/basic
-- racket\raco.exe test -l tests/openssl/https
-- racket\raco.exe test -l tests/match/main
-- racket\raco.exe test -l tests/zo-path
-- racket\raco.exe test -l tests/xml/test
-- racket\raco.exe test -c tests/stxparse
-
-notifications:
- - provider: Email
-   to: 
-    - samth@racket-lang.org
-   on_build_success: true
-   on_build_failure: true
-
- - provider: Slack
-   auth_token:
-     secure: VsZxuLzL7f/k5c/UEkiJKYxvNh9ss0Gq5ifwoZl4rlwzgtkU+2bOEo9zaP2FREF5Tb/iw4r7yQXdAYHPeo8GBQ2GQn2IksABPBEUkFrxj1k=
-   channel: notifications