Remove ios_mode::trunc from basic_ofstream openmode arguments

Remove ios_mode::trunc from basic_ofstream openmode arguments

	* include/std/fstream (basic_ifstream, basic_ofstream, basic_fstream):
	Remove outdated comments about calling c_str() to create a file stream
	from a std::string.
	(basic_ofstream::basic_ofstream, basic_ofstream::open): Remove
	redundant ios_mode::trunc bits from default arguments and comments.

From-SVN: r254226