Remove deprecated `backingStore` from KV types and tests #602

Job	Run time
lint / install (2.0.x, core)	6s
lint / install (2.0.x, jetstream)	6s
lint / install (2.0.x, kv)	8s
lint / install (2.0.x, obj)	5s
lint / install (2.0.x, services)	8s
lint / install (2.0.x, transport-deno)	8s
lint / install (2.0.x, transport-node)	5s
test-deno / install (2.0.x, core)	1m 35s
test-deno / install (2.0.x, jetstream)	1m 4s
test-deno / install (2.0.x, kv)	42s
test-deno / install (2.0.x, obj)	22s
test-deno / install (2.0.x, services)	24s
test-node / node-tests-workspace (2.0.x, 22.x)	1m 30s
test-deno / install (2.0.x, transport-deno)	13s
	6m 36s

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