Remove AppVeyor support.

Since Travis has support for Windows jobs now, the external CI services Wesnoth uses can be consolidated from AppVeyor and Travis to just Travis.

Fixes #4727

.appveyor.vs2015.yml

@@ -1,31 +0,0 @@
-version: Wesnoth-VS2015-{branch}-{build}
-clone_folder: C:\projects\wesnoth
-image: Visual Studio 2015
-configuration:
- - Release
- - Debug
-platform: win32
-skip_branch_with_pr: true
-shallow_clone: true
-matrix:
-  fast_finish: true # Stop after a build failure
-before_build:
-  - cd ..
-  - ps: wget https://github.com/aquileia/external/archive/VC14.zip -O VC14.zip
-  - 7z x VC14.ZIP
-  - ren external-VC14 external
-  - cd wesnoth
-  - set PATH=C:\projects\external\dll;%PATH%
-build_script:
-  - msbuild projectfiles\VC14\wesnoth.sln /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
-
-notifications:
-  - provider: Webhook
-    url: http://ai0867.net:6661/wesnoth
-    headers:
-      User-Agent: WesnothAppveyor/1.0
-      Version: vs2015
-      Num-Versions: 3
-      Authorization: TODO
-    on_build_success: true
-    on_build_failure: true

.appveyor.vs2017.yml

@@ -1,35 +0,0 @@
-version: Wesnoth-VS2017-{branch}-{build}
-clone_folder: C:\projects\wesnoth
-image: Visual Studio 2017
-configuration:
- - Release
- - Debug
-platform: win32
-skip_branch_with_pr: true
-shallow_clone: true
-matrix:
-  fast_finish: true # Stop after a build failure
-before_build:
-  - cd ..
-  - ps: wget https://github.com/aquileia/external/archive/VC15.zip -O VC15.zip
-  - 7z x VC15.ZIP
-  - ren external-VC15 external
-  - cd wesnoth
-  - set PATH=C:\projects\external\dll;%PATH%
-build_script:
-  - msbuild projectfiles\VC14\wesnoth.sln /property:PlatformToolset=v141 /m /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
-
-artifacts:
-  - path: wesnoth.exe
-  - path: wesnothd.exe
-
-notifications:
-  - provider: Webhook
-    url: http://ai0867.net:6661/wesnoth
-    headers:
-      User-Agent: WesnothAppveyor/1.0
-      Version: vs2017
-      Num-Versions: 3
-      Authorization: TODO
-    on_build_success: true
-    on_build_failure: true