Use J2N methods instead of StringBuilder.Remove, #664

[paulirwin]

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Use J2N methods instead of StringBuilder.Remove

Fixes #664

Description

See #664 for details. In one case, the Remove/Insert was not able to be replaced with Replace and that was noted with a comment. Also, use of the pattern sb.Remove(0, sb.Length) was replaced with sb.Clear() instead of sb.Delete(0, sb.Length), as it is more semantically understandable, less prone to mistakes, and has a few less checks internally than Delete cascading to Remove so it should be a little more efficient.

[paulirwin]

Converted to draft until OpenStringBuilder is in place