[test] [re.traits] Remove asserts failing due to invalid UTF-8

Remove the two test cases for \xDA and \xFA with UTF-8 locale, as both
characters alone are invalid in UTF-8 (short sequences).  Upon removing
them, the test passes on Linux again (and also on NetBSD, after adding
appropriate locale configuration).

Differential Revision: https://reviews.llvm.org/D55746

llvm-svn: 349378