GH-46803: [Swift] Remove swift implementation from apache/arrow after migration to new repository (#46804)

### Rationale for this change

The Swift implementation has been moved to their own repository at https://github.com/apache/arrow-swift

### What changes are included in this PR?

Remove `swift/` subfolder and github workflow for CI job.

### Are these changes tested?

No

### Are there any user-facing changes?

No but for developers of the swift implementation it has been moved to their own repository.

* GitHub Issue: #46803

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>