This flag was added to operations which have an --output-dir
command-line arguments. These operations write sstables and need a
directory where to write them. Back in the numeric-generation world this
posed a problem: if the directory contained any sstable, generation
clash was almost guaranteed, because each scylla-sstable command
invokation would start output generations from 1. To avoid this, empty
output directory was a requirement, with the
--unsafe-accept-nonempty-output-dir allowing for a force-override.
Now in the timeuuid generation days, all this is not necessary anymore:
generations are unique, so it is not a problem if the output directory
already contains sstables: the probability of generation clash is almost
0. Even if it happens, the tool will just simply fail to write the new
sstable with the clashing generation.
Remove this historic relic of a flag and the related logic, it is just a
pointless nuissance nowadays.
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