clean up CMakeList for MaterialPlugin (was ACTS-142)

Original author Christian Gumpert @cgumpert

Currently, the CMakeLists file in the MaterialPlugin still has a debug option here. This should be removed as it generates unnecessary output during the build configuration step like the one below:

{quote} -- Configuring done CMake Debug Log at Plugins/MaterialPlugins/CMakeLists.txt:17 (target_compile_options): Used compile options for target ACTSMaterialPlugin:

• -std=c++14

-- Generating done {quote}