Lucene.Net.Analysis.Fa/PersianAnalyzer: Removed PersianStemmer as per apache/lucene#904

[NightOwl888]

Reverted changes from #571 as was done in apache/lucene#904. Changed TestPersianStemFilter to use mocks.

[mocobeta]

We still have PersianStemmer in PersianAnalyzer on the main branch, it's a great addition - on the other hand, we have to consider backward compatibility on a major release (not to force users to reindex the whole data) so we can't expose the new stemmer for the default analyzer in 9.x. We will definitely have it in PerianAnalyzer in Lucene 10.0...