tests: remove some to-become-pointless ms.make_reader()-using tests
tests: remove some to-become-pointless ms.make_reader()-using tests

mutation_source are going to be created only from v2 readers and the
::make_reader() method family is scheduled for removal.

Signed-off-by: Michael Livshin <ae60993c65de9cdf2ca07b7426c976808535d555@scylladb.com>

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