AppManager: Avoid building the plugin for the 4.2 release

The appman plugin is supposed to add device support for the Qt
Application Manager. This is something which the current state of the
appman plugin does not provide, and which it will most likely do before
the release of Qt Creator 4.2.

Change-Id: I2e73e7270488097eca8cfaac2ba6ff051451f9a7
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/plugins.pro

@@ -3,7 +3,6 @@ include(../../qtcreator.pri)
 TEMPLATE  = subdirs
 
 SUBDIRS   = \
-    appmanager \
     autotest \
     clangstaticanalyzer \
     coreplugin \

src/plugins/plugins.qbs

@@ -5,7 +5,6 @@ Project {
 
     references: [
         "android/android.qbs",
-        "appmanager/appmanager.qbs",
         "autotest/autotest.qbs",
         "autotoolsprojectmanager/autotoolsprojectmanager.qbs",
         "baremetal/baremetal.qbs",