Drop cmake anti-pattern (!101)

This removes an anti-pattern from `CMakeLists.txt` (I'm very sorry for introducing it). The most annoying part about it is that it makes it difficult for a parent project (e.g. ICON) to support multi-configuration backends properly. Basically, the only right place to set `CMAKE_BUILD_TYPE` is on the command line when calling `cmake` (and generating for a single-configuration backend). Also, see [here](https://stackoverflow.com/questions/24460486/cmake-build-type-is-not-being-used-in-cmakelists-txt/24470998#24470998

).

Approved-by: Yen-Chen Chen <yen-chen.chen@kit.edu>
Merged-by: Yen-Chen Chen <yen-chen.chen@kit.edu>
Changelog: bugfix