[cmake] remove -O? from debug cxx flags

At the moment it is not trivial to configure a debug build with e.g. -O2.
Upon calling make cmake is called and the CMAKE_CXX_FLAGS_DEBUG are
appended to any given CMAKE_CXX_FLAGS, thus overriding any -O? given there.
Since the desired debug flag might vary from compiler to compiler
(-O0, -Og) anyway I propose to remove the -O? entirely from the debug
flags. Please revert or voice opinions!
[common.fixed_map] updated exceptions