Change: Remove feature flag `single-term-leader` #4299

Job	Run time
cluster-benchmark (cluster_benchmark)	52s
lint	49s
examples (stable, memstore)	40s
examples (stable, rocksstore)	6m 3s
openraft-feature-test (nightly)	57s
openraft-test-bench (nightly)	50s
examples (stable, raft-kv-memstore)	1m 25s
openraft-feature-test (nightly, serde)	45s
rt-monoio	41s
examples (stable, raft-kv-memstore-grpc)	1m 11s
openraft-feature-test (nightly, serde,singlethreaded)	45s
examples (stable, raft-kv-memstore-network-v2)	49s
stores (stores/memstore)	46s
examples (stable, raft-kv-memstore-opendal-snapshot-data)	1m 5s
test-crate-openraft (stable)	39s
test-crate-openraft (nightly)	53s
test-crate-tests (stable, 0)	40s
examples (stable, raft-kv-memstore-singlethreaded)	52s
Build (nightly, bench,serde,bt,singlethreaded)	50s
test-crate-tests (nightly, 30)	47s
examples (stable, raft-kv-rocksdb)	6m 31s
test-crate-tests (nightly, 0, single-term-leader)	49s
examples (nightly, memstore)	53s
tests-feature-test (nightly)	48s
examples (nightly, rocksstore)	6m 13s
tests-feature-test (nightly, single-term-leader)	46s
examples (nightly, raft-kv-memstore)	1m 19s
examples (nightly, raft-kv-memstore-grpc)	1m 13s
examples (nightly, raft-kv-memstore-network-v2)	49s
examples (nightly, raft-kv-memstore-opendal-snapshot-data)	1m 7s
examples (nightly, raft-kv-memstore-singlethreaded)	53s
examples (nightly, raft-kv-rocksdb)	6m 34s
	50m 14s

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