Fix subcommand override edge case

[brentyi]

This addresses an edge case when we:

• have a default value in the config class
• override that default value with a subcommand annotation
• override it again with a default instance

The final override here was failing due to some redundant logic in handling subcommand configs.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (bc55c77) 99.58% compared to head (7354864) 99.58%.

:exclamation: Current head 7354864 differs from pull request most recent head 0128184. Consider uploading reports for the commit 0128184 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #117 +/- ## ======================================= Coverage 99.58% 99.58% ======================================= Files 24 24 Lines 1926 1930 +4 ======================================= + Hits 1918 1922 +4 Misses 8 8 ``` | [Flag](https://app.codecov.io/gh/brentyi/tyro/pull/117/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brent+Yi) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/brentyi/tyro/pull/117/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brent+Yi) | `99.58% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brent+Yi#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.