tests/multishard_mutation_query_test: drop range tombstone test
tests/multishard_mutation_query_test: drop range tombstone test

Soon a much better test will be added that will also cover range
tombstones and much more.

avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla