Remove `ProspectiveParachainsMode` usage in backing subsystem #5924

Job	Run time
preflight / preflight	41s
preflight / ci-versions	1m 4s
test-full-crypto-feature	2m 42s
cargo-check-benches (master)	21m 13s
cargo-check-benches (current)	22m 10s
cargo-check-each-crate (1)	19m 11s
cargo-check-each-crate (2)	20m 2s
cargo-check-each-crate (3)	9m 29s
cargo-check-each-crate (4)	17m 35s
cargo-check-each-crate (5)	25m 18s
cargo-check-each-crate (6)	15m 15s
cargo-check-each-crate (7)	30m 9s
test-frame-ui	46m 23s
test-node-metrics	10m 32s
cargo-check-all-crate-macos	11m 40s
test-frame-examples-compile-to-wasm	3m 56s
test-deterministic-wasm	6m 32s
check-tracing	2m 29s
check-metadata-hash	6m 27s
node-bench-regression-guard	14s
All test misc tests passed	1s
	4h 33m 3s

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