runtime: cleanup legacy tests of alt multi-threaded runtime

[ADD-SP]

The alt multi-threaded runtime and relevant tests were initially added in #5823, and the alt multi-threaded runtime was removed in #7275.

However, there are two test combinations in tokio/tests/rt_common.rs that should have been deleted in #7275 were not.

[Darksonn]

Thanks. There is also:

tokio/.github/workflows/loom.yml

Lines 99 to 122 in aff24df

	loom-multi-thread-alt:
	name: loom ALT multi-thread scheduler
	# base_ref is null when it's not a pull request
	if: github.repository_owner == 'tokio-rs' && (contains(github.event.pull_request.labels.*.name, 'R-loom-multi-thread-alt') || (github.base_ref == null))
	runs-on: ubuntu-latest
	strategy:
	matrix:
	include:
	- scope: loom_multi_thread_alt::group_a
	- scope: loom_multi_thread_alt::group_b
	- scope: loom_multi_thread_alt::group_c
	- scope: loom_multi_thread_alt::group_d
	steps:
	- uses: actions/checkout@v4
	- name: Install Rust ${{ env.rust_stable }}
	uses: dtolnay/rust-toolchain@master
	with:
	toolchain: ${{ env.rust_stable }}
	- uses: Swatinem/rust-cache@v2
	- name: loom ${{ matrix.scope }}
	run: cargo test --lib --release --features full -- --nocapture $SCOPE
	working-directory: tokio
	env:
	SCOPE: ${{ matrix.scope }}