Cleanup: Delete dead code from send_merge_thread()

range is always deferenced before it reaches this check, such that the
kmem_zalloc() call is never executed.

There is also no need to set range->eos_marker = B_TRUE because it is
already set.

Coverity incorrectly complained about a potential NULL pointer
dereference because of this.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14042