Do not clear aggregated contents of TextBuffer when releaseBuffers() called

[cowtowncoder]

(background issue: FasterXML/jackson-databind#2635)

When a JsonParser is closed, buffers associated with it are deallocated, since some of them are recycled. This works fine when caller controls closing directly, but there are cases where this occurs automatically -- for example, when Jackson databind ObjectMapper manages process, when it has implicitly created parser. In such cases, there one specific workflow path where some of the contents of most recently decoded token are needed, see:

FasterXML/jackson-databind#2635

Because of this, upon close, it would make sense NOT to clear _resultString.
(in theory, we might want full contents, but that could have issues with prevent recycling of buffers and so on -- for immediate problem, we know that _resultString has been aggregated and nothing more is needed).