Replace snowballstemmer with a dummy implementation

This change removes the dependency on snowballstemmer in a way that it's replaced by a dummy implementation (doing nothing) without a significant impact on the search feature in the generated documentation.

I did an impact check in COPR: https://copr.devel.redhat.com/coprs/lbalhar/sphinx-snowball/builds/ None of the failed builds are caused by this change.

Then I installed 4 randomly selected documentation packages and tested the generated documentation and it works fine including the search feature returning meaningful results.