CI - Windows - Back to nmake

... free CI does only offer one core, so no need to install Jom - back to nmake

appveyor.yml

@@ -18,13 +18,12 @@ init:
 
 install:
 - choco install winflexbison
-- choco install jom
 - copy qwt\qwtconfig.pri.in qwt\qwtconfig.pri
 - copy c:\libs\gcconfig64-Release.appveyor.pri src\gcconfig.pri
 
 build_script:
 - qmake.exe build.pro -r -spec win32-msvc2015
-- C:\ProgramData\chocolatey\lib\jom\tools\jom -j 2
+- nmake
 
 #notifications:
 #- provider: GitHubPullRequest