Don't set DCMAKE_BUILD_TYPE in kf5-frameworks-build-include

Summary:
In kde/src/log/2018-12-01-04/kconfig/cmake.log, I found this:

# kdesrc-build running: 'cmake' '/home/gregor/kde/src/frameworks/kconfig' '-DCMAKE_CXX_COMPILER_LAUNCHER=ccache' '-GNinja' '-DCMAKE_BUILD_TYPE=Debug' '-DBUILD_TESTING=TRUE' '-DCMAKE_BUILD_TYPE:STRING=debug' '-DCMAKE_CXX_FLAGS:STRING=-pipe ' '-DCMAKE_INSTALL_PREFIX=/home/gregor/kde/usr' '-DCMAKE_PREFIX_PATH=/usr'
# from directory: /home/gregor/kde/build/frameworks/kconfig
-- The C compiler identification is GNU 8.2.1
-- The CXX compiler identification is GNU 8.2.1
...

Note, that CMAKE_BUILD_TYPE from my global .kdesrc-buildrc gets overridden from "Debug" to "debug". For me, this came surprising (also the lowercase "debug" is less common than the capital D).

This change removes the definition of CMAKE_BUILD_TYPE from kf5-frameworks-build-include.

Reviewers: mpyne, dfaure

Reviewed By: dfaure

Differential Revision: https://phabricator.kde.org/D17290