fix: remove `Subtype.instInhabited`

[kmill]

It has had a long history going back 10 years, but its time has come to an end since this instance is never applicable.

[leanprover-community-mathlib4-bot]

Mathlib CI status (docs):

• ❌ Mathlib branch lean-pr-testing-4055 built against this PR, but linting failed. (2024-05-02 20:55:37) View Log
• 🟡 Mathlib branch lean-pr-testing-4055 build against this PR was cancelled. (2024-05-02 21:13:13) View Log
• ❌ Mathlib branch lean-pr-testing-4055 built against this PR, but lean4checker failed. (2024-05-02 22:13:18) View Log
• 🟡 Mathlib branch lean-pr-testing-4055 build this PR didn't complete normally. (2024-05-02 22:40:06) View Log
• ❗ Mathlib CI can not be attempted yet, as the nightly-testing-2024-05-02 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-mathlib, Mathlib CI should run now. (2024-05-03 02:07:24)
• 💥 Mathlib branch lean-pr-testing-4055 build failed against this PR. (2024-05-03 02:08:03) View Log
• 💥 Mathlib branch lean-pr-testing-4055 build failed against this PR. (2024-05-03 11:02:47) View Log
• ✅ Mathlib branch lean-pr-testing-4055 has successfully built against this PR. (2024-05-03 13:04:03) View Log
• ✅ Mathlib branch lean-pr-testing-4055 has successfully built against this PR. (2024-05-03 13:47:56) View Log

[kmill]

!bench

[leanprover-bot]

Here are the benchmark results for commit 24b472541c2fdb91fe573654b095aef50b46f6ed. There were no significant changes against commit b470eb522bfd68ca96938c23f6a1bce79da8a99f.