initerrors(): Remove this function. String-based standard exceptions

are no longer supported (i.e. -X option is removed).

_PyBuiltin_Init_1(): Don't call initerrors().  This does mean that it
is possible to raise an ImportError before that exception has been
initialized, say because exceptions.py can't be found, or contains
bogosity.  See changes to errors.c for how this is handled.

_PyBuiltin_Init_2(): Don't test Py_UseClassExceptionsFlag, just go
ahead and initialize the class-based standard exceptions.  If this
fails, we throw a Py_FatalError.