remove unreachable break statements

Found with clang's -Wunreachable-code-break

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/builtin_function.cpp

@@ -164,7 +164,6 @@ static int parse_cmd_opts(function_cmd_opts_t &opts, int *optind,  //!OCLINT(hig
             }
             default: {
                 DIE("unexpected retval from wgetopt_long");
-                break;
             }
         }
     }