Revert "Add a test case for dmu_free_long_range() to ztest"

This reverts commit d0de2e82df579f4e4edf5643b674a1464fae485f which
introduced a new test case to ztest which is failing occasionally
during automated testing. The change is being reverted until
the issue can be fully investigated.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #4754