this is a part of a series to migrating from `operator<<(ostream&, ..)`
based formatting to fmtlib based formatting. the goal here is to enable
fmtlib to print `range_tombstone_list` and `range_tombstone_entry`
without the help of `operator<<`.
the corresponding `operator<<()` for `range_tombstone_entry` is moved
into test, where it is used. and the other one is dropped in this change,
as all its callers are now using fmtlib for formatting now.
Refs #13245
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#13627