cleanup: remove legacy backend and npm-app #5059

Job	Run time
build-and-check	2m 1s
test-backend	40s
test-integration-common	39s
test-packages/internal	39s
test-web	48s
test-cli	1m 7s
test-packages/agent-runtime	55s
test-npm-app	40s
test-sdk	53s
test-.agents	40s
test-integration-cli	41s
test-integration-packages/billing	42s
test-integration-packages/agent-runtime	39s
test-integration-.agents	1m 12s
test-common	40s
test-integration-backend	39s
test-integration-web	38s
test-integration-packages/internal	39s
test-integration-sdk	53s
test-integration-npm-app	39s
test-packages/billing	38s
	17m 2s

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