Remove instances of solr.hdfs.blockcache.write.enabled from tests and docs

solr.hdfs.blockcache.write.enabled is currently disabled whether or not you set it in the solr configs. It makes sense to just avoid mentioning it in the docs (it's still there on https://cwiki.apache.org/confluence/display/solr/Running+Solr+on+HDFS) and the morphlines examples. Best case, it's unnecessary information. Worse case, it causes people trouble on versions where it's not disabled.