Change: Remove feature flag `single-term-leader` #4300

Job	Run time
openraft-test-bench (nightly)	59s
cluster-benchmark (cluster_benchmark)	47s
openraft-feature-test (nightly)	48s
openraft-feature-test (nightly, serde)	51s
rt-monoio	47s
stores (stores/memstore)	51s
openraft-feature-test (nightly, serde,singlethreaded)	50s
test-crate-openraft (stable)	47s
test-crate-tests (stable, 0)	3m 4s
test-crate-openraft (nightly)	52s
Build (nightly, bench,serde,bt,singlethreaded)	50s
test-crate-tests (nightly, 30)	3m 13s
test-crate-tests (nightly, 0, single-term-leader)	2m 57s
tests-feature-test (nightly)	3m 2s
examples (stable, memstore)	44s
tests-feature-test (nightly, single-term-leader)	2m 59s
lint	1m 44s
examples (stable, rocksstore)	6m 39s
examples (stable, raft-kv-memstore)	2m 35s
examples (stable, raft-kv-memstore-grpc)	2m 13s
examples (stable, raft-kv-memstore-network-v2)	57s
examples (stable, raft-kv-memstore-opendal-snapshot-data)	1m 20s
examples (stable, raft-kv-memstore-singlethreaded)	50s
examples (stable, raft-kv-rocksdb)	8m 23s
examples (nightly, memstore)	1m 2s
examples (nightly, rocksstore)	11m 35s
examples (nightly, raft-kv-memstore)	2m 22s
examples (nightly, raft-kv-memstore-grpc)	1m 47s
examples (nightly, raft-kv-memstore-network-v2)	1m 15s
examples (nightly, raft-kv-memstore-opendal-snapshot-data)	1m 26s
examples (nightly, raft-kv-memstore-singlethreaded)	1m 13s
examples (nightly, raft-kv-rocksdb)	12m 50s
	1h 22m 32s

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