Drop appveyor test job

We've moved to Github Actions so we'll proceed with dropping the
Appveyor job and updating the docs accordingly.

We'll no longer publish debug artifacts. Users may fetch
the Ceph bundle instead, which includes a WNBD build signed
by Cloudbase: https://cloudbase.it/ceph-for-windows/

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>

README.md

@@ -4,7 +4,7 @@ Windows Network Block Device (WNBD)
 Build Status:
 -------------
 
-[![Build status](https://ci.appveyor.com/api/projects/status/4sh808y8v3hv0k2d?svg=true)](https://ci.appveyor.com/project/petrutlucian94/wnbd)
+![Build status](https://github.com/cloudbase/wnbd/actions/workflows/pr-tests.yaml/badge.svg)
 
 What is WNBD?
 -------------
@@ -91,15 +91,6 @@ copy wnbd\vstudio\x64\Debug\libwnbd.dll $outDir
 copy wnbd\vstudio\reinstall.ps1 $outDir
 ```
 
-You can also download the latest prebuilt packages from Appveyor via the links:
-
-* [Debug](https://ci.appveyor.com/api/projects/aserdean/wnbd/artifacts/wnbd-Debug.zip?job=Configuration%3A+Debug)
-* [Release](https://ci.appveyor.com/api/projects/aserdean/wnbd/artifacts/wnbd-Release.zip?job=Configuration%3A+Release)
-
-When using the above Appveyor artifacts, make sure to also install the latest
-Microsoft VC runtime. Note that VS 2015, 2017 and 2019 use the same redistributable
-VC runtime package. You can skip this step when using the below MSI installer.
-
 [This project](https://github.com/cloudbase/ceph-windows-installer) allows building
 an MSI installer that bundles WNBD and the Ceph Windows clients.