Change: Remove feature flag `single-term-leader` #4293

Job	Run time
Build (nightly, bench,serde,bt,singlethreaded)	51s
lint	1m 48s
cluster-benchmark (cluster_benchmark)	49s
openraft-feature-test (nightly)	51s
openraft-feature-test (nightly, serde)	49s
examples (stable, memstore)	51s
openraft-test-bench (nightly)	1m 2s
openraft-feature-test (nightly, serde,singlethreaded)	52s
rt-monoio	43s
examples (stable, rocksstore)	6m 11s
examples (stable, raft-kv-memstore)	2m 31s
stores (stores/memstore)	54s
test-crate-openraft (stable)	55s
examples (stable, raft-kv-memstore-grpc)	59s
test-crate-tests (stable, 0)	3m 2s
test-crate-openraft (nightly)	49s
examples (stable, raft-kv-memstore-network-v2)	55s
test-crate-tests (nightly, 30)	3m 9s
tests-feature-test (nightly)	2m 59s
examples (stable, raft-kv-memstore-opendal-snapshot-data)	1m 12s
test-crate-tests (nightly, 0, single-term-leader)	2m 58s
tests-feature-test (nightly, single-term-leader)	2m 56s
examples (stable, raft-kv-memstore-singlethreaded)	57s
examples (stable, raft-kv-rocksdb)	8m 15s
examples (nightly, memstore)	1m 2s
examples (nightly, rocksstore)	11m 49s
examples (nightly, raft-kv-memstore)	2m 20s
examples (nightly, raft-kv-memstore-grpc)	1m 6s
examples (nightly, raft-kv-memstore-network-v2)	1m 10s
examples (nightly, raft-kv-memstore-opendal-snapshot-data)	1m 24s
examples (nightly, raft-kv-memstore-singlethreaded)	1m 15s
examples (nightly, raft-kv-rocksdb)	13m 4s
	1h 20m 28s

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