Clean up handling of special float values in string.format.

library/string.m:
	Delete old code related to the handling of float special
	values in the Mercury implementation of string.format.

	Delete a commented added in my previous change that is
	incorrect.
Fix printing of special float values in C# grade.

library/string.m:
	In the C# grade, do not append ".0" to a special float
	value when converting it to a string.

tests/hard_coded/write_special_float.exp3:
	Alternative expected output for this for use with the
	Java and C# grades.

NEWS:
	Announce the fix.
Merge remote-tracking branch 'upstream/version-14_01-branch' into feature-char-lead-trail-surrogate