Appveyor: Remove /m to attempt to resolve build failures

/m also doesn't seem to have made the builds any faster.

Author: jagerman

.appveyor.yml

@@ -40,6 +40,6 @@ install:
 build_script:
 - cmake -G "%CMAKE_GENERATOR%" -A "%CMAKE_ARCH%" -DPYBIND11_WERROR=ON
 - set MSBuildLogger="C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
-- cmake --build . --config Release --target pytest -- /v:m /m /logger:%MSBuildLogger%
-- cmake --build . --config Release --target test_cmake_build -- /v:m /m /logger:%MSBuildLogger%
+- cmake --build . --config Release --target pytest -- /v:m /logger:%MSBuildLogger%
+- cmake --build . --config Release --target test_cmake_build -- /v:m /logger:%MSBuildLogger%
 on_failure: if exist "tests\test_cmake_build" type tests\test_cmake_build\*.log