build: do not print the default value of --c++-standard in help output

[tchaikov]

before this change, the output of ./configure.py --help looks like:

  --c++-standard CPP_STANDARD
                        C++ standard to build with [default: ]

because the default value of --c++-standard option is an empty string. printing out its value is not helpful, and could be confusing.

as the actual default value of this option varies depending on the used compiler, so even if we print out the deduced C++ standard, it is still confusing. not to mention that argparse does not differentiate the defaulted value in the returned namespace from parse_args() from the user specified one. so, a simpler approach is just do not specify the default value.

in this change, we

• do not specify the default value for --c++-standard, so its value would be None if not specified
• do not print its default value in the --help output

[tchaikov]

@scylladb/seastar-maint hello maintainers, could you help review this change?