cleanup: remove legacy backend and npm-app #5053

Job	Run time
build-and-check	2m 6s
test-integration-cli	37s
test-integration-backend	42s
test-integration-npm-app	38s
test-integration-packages/billing	47s
test-integration-web	46s
test-integration-sdk	56s
test-integration-packages/internal	48s
test-integration-common	37s
test-cli	1m 6s
test-integration-packages/agent-runtime	38s
test-common	41s
test-packages/internal	41s
test-.agents	39s
test-packages/agent-runtime	56s
test-packages/billing	47s
test-backend	38s
test-integration-.agents	1m 8s
test-sdk	54s
test-npm-app	38s
test-web	43s
	17m 26s

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