libstdc++: Remove std_ratio_t_tuple

This removes the std_ratio_t_tuple function from the Python
pretty-printer code.  It is not used.  Apparently the relevant parts
were moved to StdChronoDurationPrinter._ratio at some point in the
past.

libstdc++-v3/ChangeLog:

	* python/libstdcxx/v6/printers.py (std_ratio_t_tuple):
	Remove.