[PYOPENMS,FIX] fixed regression from last commit e334d31
[PYOPENMS,CMAKE] running cmake removes pyOpenMS/build/ dir if exists

    When working on pyopenms existing shared libraries in pyOpenMS/build
    might be out of date, but the linker prefers these and possibly
    complains about "undefined symbols".
    This commit adapts pyopenms.cmake such that running "cmake" this
    build/ is removed in the OpenMS build tree if it exists.