Remove unused self.selected_archives variable in ArchiveTab (#2388)

[mr-raj12]

Description

Remove self.selected_archives = None from ArchiveTab.__init__() in src/vorta/views/archive_tab.py. This instance variable was initialized but never read
anywhere in the codebase. It already had a # TODO: remove unused variable comment.

Note: The local variable selected_archives used elsewhere in the same file (lines 532, 973) is unrelated — those are local variables, not the instance
attribute.

Related Issue

Closes #2388

Motivation and Context

Dead code cleanup. The variable was write-only and flagged with a TODO for removal.

How Has This Been Tested?

• Ruff lint and format checks pass
• Unit tests (misc, schedule, source) pass with no regressions
• Single-line deletion with no references — no functional impact

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have read the CONTRIBUTING guide.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

I provide my contribution under the terms of the license of this repository and I affirm the Developer Certificate of Origin.