test: Clean up tests `gettime` is outdated

## Summary

The `gettime` test is outdated:

- It was originally added in Bitcoin Core to verify the time handling in the MinGW toolchain and detect 32-bit vs 64-bit mismatches. However, the code now relies on std::chrono::system_clock, making the original check unnecessary.
- Although `system_clock` could still return incorrect values, any such issue would likely affect all
- `GetTime<>` calls, not just those with second-level precision. Such errors would probably trigger signed integer overflows at nanosecond precision, which should either be caught by UBSan or by the assert(`ret > 0s`) check. If not, the issue would become obvious in the debug log at startup.

## Notes

- For now, the test is can be removed. Alternatively, it could be expanded to cover time handling more comprehensively and updated alongside the block header timestamp logic.
- However, since that timestamp won’t exceed the year 2106, this is already covered by the `_test_y2106` functional test.

Search the test suite for the outdated `gettime` test and inspect the `_test_y2106` functional test mentioned in the issue. Confirm that the existing functional coverage makes the old check unnecessary, then remove or update the test as appropriate and run the affected tests.

Written by the indexing model from the issue text.