Remove temporary eval history configuration

[OskarEichler]

Summary

Delete the same temporary configuration key that eval-history initialization inserts.

Reproduction

Initialization writes IRB.conf[:__TMP__EHV__] but deletes the mistyped :__TMP_EHV__, leaving the history object in global configuration after the workspace binding is established.

Verification

• current and cumulative candidate suites: 402 tests / 2,488 assertions, zero failures/errors and three omissions
• all 111 Ruby files pass syntax
• RuboCop checks 113 files with no offenses
• RDoc and Rails 8.1.3.1 loading pass

Compatibility

Removes only an internal temporary key after its intended one-shot use. Prepared with AI-assisted source review; no repository tests were changed.

[tompng]

Nice catch 👍