cleanup: remove legacy backend and npm-app #5065

Job	Run time
build-and-check	2m 4s
test-integration-common	39s
test-integration-.agents	1m 7s
test-integration-npm-app	38s
test-integration-cli	47s
test-integration-backend	40s
test-cli	1m 6s
test-backend	41s
test-integration-sdk	58s
test-integration-web	41s
test-packages/internal	35s
test-integration-packages/billing	40s
test-integration-packages/internal	40s
test-sdk	56s
test-integration-packages/agent-runtime	42s
test-npm-app	45s
test-packages/agent-runtime	50s
test-common	42s
test-packages/billing	39s
test-.agents	47s
test-web	47s
	17m 24s

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