Change: Remove feature flag `single-term-leader` #4298

Job	Run time
lint	1m 45s
examples (stable, memstore)	46s
cluster-benchmark (cluster_benchmark)	49s
examples (stable, rocksstore)	6m 10s
examples (stable, raft-kv-memstore)	2m 42s
openraft-test-bench (nightly)	1m 0s
openraft-feature-test (nightly)	50s
examples (stable, raft-kv-memstore-grpc)	1m 47s
rt-monoio	44s
openraft-feature-test (nightly, serde)	52s
examples (stable, raft-kv-memstore-network-v2)	48s
openraft-feature-test (nightly, serde,singlethreaded)	52s
stores (stores/memstore)	53s
examples (stable, raft-kv-memstore-opendal-snapshot-data)	1m 3s
test-crate-openraft (stable)	46s
examples (stable, raft-kv-memstore-singlethreaded)	54s
test-crate-openraft (nightly)	52s
Build (nightly, bench,serde,bt,singlethreaded)	53s
examples (stable, raft-kv-rocksdb)	8m 24s
test-crate-tests (stable, 0)	3m 4s
test-crate-tests (nightly, 30)	3m 8s
examples (nightly, memstore)	54s
tests-feature-test (nightly)	3m 2s
test-crate-tests (nightly, 0, single-term-leader)	3m 7s
tests-feature-test (nightly, single-term-leader)	2m 56s
examples (nightly, rocksstore)	11m 29s
examples (nightly, raft-kv-memstore)	2m 20s
examples (nightly, raft-kv-memstore-grpc)	1m 43s
examples (nightly, raft-kv-memstore-network-v2)	1m 9s
examples (nightly, raft-kv-memstore-opendal-snapshot-data)	1m 33s
examples (nightly, raft-kv-memstore-singlethreaded)	1m 7s
examples (nightly, raft-kv-rocksdb)	12m 32s
	1h 20m 54s

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