AppVeyor: don't cache cygwin packages, they are over cache size limit

anyway.

.appveyor.yml

@@ -4,8 +4,6 @@
 # TODO: test msvc
 version: 0.0.0.0.1-branch-{branch}-build-{build}
 clone_depth: 10
-cache:
-    - c:\cygwin-setup-cache -> .appveyor.yml
 environment:
     matrix:
         - cygwin_url: https://cygwin.com/setup-x86_64.exe