Remove unused self.selected_archives variable in ArchiveTab

[mr-raj12]

The problem

self.selected_archives = None is initialized in ArchiveTab.__init__()
(src/vorta/views/archive_tab.py, line 157) but is never read anywhere in
the codebase. It already has a # TODO: remove unused variable comment.
The local variable selected_archives used elsewhere in the same file
(lines 532, 973) is unrelated — those are local variables, not the instance attribute.

Requested Solution

Delete line 157 (self.selected_archives = None # TODO: remove unused variable).
No other changes needed since nothing references the instance variable.

Alternatives

None — straightforward dead code removal.

Additional context

Low priority, 1-line fix. Could be a good first issue for new contributors.