libstdc++: Do not use std::vector<bool>::reference default ctor [PR115098]

This default constructor was made private by r15-3124-gb25b101bc38000 so
the pretty printer tests need a fix to stop using it. There's no
conforming way to get a default-constructed 'reference' now, e.g. trying
to access an element of a default-constructed std::vector<bool> will
trigger an assertion. Remove the tests, but leave a comment in the
printer code about handling it.

libstdc++-v3/ChangeLog:

	PR libstdc++/115098
	* python/libstdcxx/v6/printers.py (StdBitReferencePrinter): Add
	comment.
	* testsuite/libstdc++-prettyprinters/simple.cc: Do not default
	construct std::vector<bool>::reference.
	* testsuite/libstdc++-prettyprinters/simple11.cc: Likewise.