Un-blacklist core/txns from sharding test suites

This blacklist seems to have been left over from MongoDB 4.0. As of MongoDB 4.2, it should now be able to be un-blacklisted. Similar blacklists exist for the following suites:
buildscripts/resmokeconfig/suites/causally_consistent_jscore_passthrough.yml
buildscripts/resmokeconfig/suites/causally_consistent_jscore_passthrough_auth.yml
buildscripts/resmokeconfig/suites/logical_session_cache_sharding_100ms_refresh_jscore_passthrough.yml
buildscripts/resmokeconfig/suites/logical_session_cache_sharding_10sec_refresh_jscore_passthrough.yml
buildscripts/resmokeconfig/suites/logical_session_cache_sharding_1sec_refresh_jscore_passthrough.yml
buildscripts/resmokeconfig/suites/logical_session_cache_sharding_default_refresh_jscore_passthrough.yml
buildscripts/resmokeconfig/suites/sharded_causally_consistent_jscore_passthrough.yml
buildscripts/resmokeconfig/suites/sharding_jscore_op_query_passthrough.yml
buildscripts/resmokeconfig/suites/sharding_jscore_passthrough.yml