<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def delete(build_folder):
 """Delete build directory and all its contents. """

    if _meta_.del_build in ["on", "ON"] and os.path.exists(build_folder):
        shutil.rmtree(build_folder)