chore(ci): remove Win from Cirrus builds (#2689)

Remove Cirrus Win targets until there's a good resolution.

Postmortem:
- CirrusCI uses [Windows Server
Containers](https://docs.microsoft.com/en-us/virtualization/windowscontainers/about/#windows-container-types) to run windows builds.
- Windows Server Containers don't include all the libraries that are required to run general application. See [msdn discussion](https://social.msdn.microsoft.com/Forums/en-US/a50e7dd7-c73f-45e0-89b4-92ba3ad2bcda/ieframedll-and-shdocvwdll-are-missing-from-windowsservercore-image?forum=windowscontainers).
- A recent [chromium commit](https://crrev.com/560677) added win dependencies missing in Windows Server Containers.

We'll be able to bring back Win builds on Cirrus once the full-fledged
Win containers are released (announced
[recently](https://channel9.msdn.com/Events/Build/2018/BRK2149).

.ci/node6/Dockerfile.windows

@@ -1,12 +0,0 @@
-FROM microsoft/windowsservercore:latest
-
-ENV NODE_VERSION 6.12.3
-
-RUN setx /m PATH "%PATH%;C:\nodejs"
-
-RUN powershell -Command \
-    netsh interface ipv4 show interfaces ; \
-    netsh interface ipv4 set subinterface 18 mtu=1460 store=persistent ; \
-    Invoke-WebRequest $('https://nodejs.org/dist/v{0}/node-v{0}-win-x64.zip' -f $env:NODE_VERSION) -OutFile 'node.zip' -UseBasicParsing ; \
-    Expand-Archive node.zip -DestinationPath C:\ ; \
-    Rename-Item -Path $('C:\node-v{0}-win-x64' -f $env:NODE_VERSION) -NewName 'C:\nodejs'

.ci/node7/Dockerfile.windows

@@ -1,11 +0,0 @@
-FROM microsoft/windowsservercore:latest
-
-ENV NODE_VERSION 7.10.1
-
-RUN setx /m PATH "%PATH%;C:\nodejs"
-
-RUN powershell -Command \
-    netsh interface ipv4 set subinterface 18 mtu=1460 store=persistent ; \
-    Invoke-WebRequest $('https://nodejs.org/dist/v{0}/node-v{0}-win-x64.zip' -f $env:NODE_VERSION) -OutFile 'node.zip' -UseBasicParsing ; \
-    Expand-Archive node.zip -DestinationPath C:\ ; \
-    Rename-Item -Path $('C:\node-v{0}-win-x64' -f $env:NODE_VERSION) -NewName 'C:\nodejs'

.cirrus.yml

@@ -3,11 +3,6 @@ env:
 
 task:
   matrix:
-    - name: node6 (windows)
-      allow_failures: true
-      windows_container:
-        dockerfile: .ci/node6/Dockerfile.windows
-        os_version: 2016
     - name: node6 (linux)
       container:
         dockerfile: .ci/node6/Dockerfile.linux
@@ -17,11 +12,6 @@ task:
 
 task:
   matrix:
-    - name: node7 (windows)
-      allow_failures: true
-      windows_container:
-        dockerfile: .ci/node7/Dockerfile.windows
-        os_version: 2016
     - name: node7 (linux)
       container:
         dockerfile: .ci/node7/Dockerfile.linux