Revert "[mono][interp] Preserve nullable boxing for GetType"

[jkotas]

Reverts #132732

Contributes to #133207. The new test is failing on many CoreCLR legs.

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[Copilot]

🟡 Changes recommended

The reverted optimization changes observable Nullable<T>.GetType() behavior in the interpreter and the revert also removes innerloop regression coverage for this scenario.

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Pull request overview

This PR reverts prior changes related to how the Mono interpreter recognizes and optimizes box + GetType() patterns, and it removes an accompanying regression test for boxed Nullable<T>.GetType() behavior.

Changes:

• Removes the Nullable<T> exclusion from the interpreter’s box + object.GetType() peephole in transform.c.
• Removes a non-[OuterLoop] xUnit regression test in gettype.cs (leaving only an [OuterLoop] entrypoint test).
• Minor using-directive reshuffle/cleanup in the test file.

File summaries

File	Description
src/mono/mono/mini/interp/transform.c	Re-enables the box+GetType folding for nullable types by removing the nullable guard.
src/tests/JIT/Directed/nullabletypes/gettype.cs	Drops the non-[OuterLoop] regression test that asserted correct boxed-nullable GetType() semantics.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 2
• Review effort level: Lite