Reusing SimpleTVPostingsEnum instance instead of creating new one.

### Description

Hello everyone,
I was going through the code of lucene and found a small #TODO where we are creating a new class instance instead of reusing an old variable.

https://github.com/apache/lucene/blob/b843b4c6c395cff92b72997c8f59b0579d68365d/lucene/codecs/src/java/org/apache/lucene/codecs/simpletext/SimpleTextTermVectorsReader.java#L411

I thought why not make a first contribution and learn something new.

Read lucene/codecs/src/java/org/apache/lucene/codecs/simpletext/SimpleTextTermVectorsReader.java around line 411 and inspect the TODO and surrounding variable usage. Run the relevant Lucene tests for the simpletext term-vectors reader; done means the existing SimpleTVPostingsEnum is reused and the tests pass.

Written by the indexing model from the issue text.