tests: mutation_source_tests: Unroll run_flat_mutation_reader_tests()
tests: mutation_source_tests: Unroll run_flat_mutation_reader_tests()

All readers are now flat so there is no need for this grouping.

Will be needed for the next patch, which needs a single function with
all test cases.

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