Remove the no-op AppVeyor with_win_header job

The with_win_header matrix entry patched Windows.h into
single_include/nlohmann/json.hpp before building, but JSON_MultipleHeaders
has defaulted to ON since #3532 (2022-06), so CMakeLists.txt points the
tests at include/ and the patched single header is never compiled. The
job has been a no-op VS2015 build since then.

Windows.h coverage already exists through tests/src/unit-windows_h.cpp
(#3631), which runs in every MSVC job. Delete the dead matrix entry and
its before_build steps, and cite unit-windows_h.cpp from the QA page.

Part of #5715

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/external_ci/appveyor.yml

@@ -30,14 +30,6 @@ environment:
       CMAKE_OPTIONS: ""
       GENERATOR: Visual Studio 14 2015
 
-    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-      configuration: Release
-      platform: x86
-      name: with_win_header
-      CXX_FLAGS: "/W4 /WX"
-      CMAKE_OPTIONS: ""
-      GENERATOR: Visual Studio 14 2015
-
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       configuration: Release
       platform: x86
@@ -74,9 +66,6 @@ install:
   - if "%platform%"=="x86"    set GENERATOR_PLATFORM=Win32
 
 before_build:
-  # for with_win_header build, inject the inclusion of Windows.h to the single-header library
-  - ps: if ($env:name -Eq "with_win_header") { $header_path = "single_include\nlohmann\json.hpp" }
-  - ps: if ($env:name -Eq "with_win_header") { "#include <Windows.h>`n" + (Get-Content $header_path | Out-String) | Set-Content $header_path }
   - cmake . -G "%GENERATOR%" -A "%GENERATOR_PLATFORM%" -DCMAKE_CXX_FLAGS="%CXX_FLAGS%" -DCMAKE_IGNORE_PATH="C:/Program Files/Git/usr/bin" -DJSON_BuildTests=On "%CMAKE_OPTIONS%"
 
 build_script:

docs/mkdocs/docs/community/quality_assurance.md

@@ -132,7 +132,8 @@ Note: Some modern features (like C++20 ranges or filesystem support) may be disa
   [libstdc++](https://gcc.gnu.org/onlinedocs/libstdc++/) to detect subtle differences or incompatibilities.
 - [x] The code checked with [Include What You Use (IWYU)](https://include-what-you-use.org) that all required standard
   headers are included.
-- [x] On Windows, the library is compiled with `<Windows.h>` being included to detect and avoid common bugs.
+- [x] On Windows, the library is compiled with `<Windows.h>` being included to detect and avoid common bugs (see
+  [`unit-windows_h.cpp`](https://github.com/nlohmann/json/blob/develop/tests/src/unit-windows_h.cpp)).
 - [x] The library is compiled with exceptions disabled to support alternative means of error handling.
 
 ## Stable public API