Remove no longer needed optimization

[wendigo]

Since FasterXML/jackson-core#1081 this optimization does no longer make sense as it's applied internally by Jackson if needed.

Alternative to #22941

[Dith3r]

Flow with InputStreamReader cannot trigger optimization with StringReader, and additionally it creates 8kb Heap byte buffer for every Slice. JFRs and benchmarks were created with Jackson with patch and there is still difference 50% in CPU time with and without this part of code.