use centralized handling of QT_BUILD_PARTS

Change-Id: I220d36267e36bd2993fc520dea1fdffa3405b0a5
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>

qttools.pro

-TEMPLATE = subdirs
-
-module_qttools_src.subdir = src
-module_qttools_src.target = module-qttools-src
-
-module_qttools_tests.subdir = tests
-module_qttools_tests.target = module-qttools-tests
-module_qttools_tests.depends = module_qttools_src
-module_qttools_tests.CONFIG = no_default_install
-!contains(QT_BUILD_PARTS,tests):module_qttools_tests.CONFIG += no_default_target
-
-SUBDIRS += module_qttools_src \
-           module_qttools_tests \
+# FIXME: the src directory contains both libs and tools parts
+load(qt_parts)