cleanup: remove legacy backend and npm-app (#389) #5080

Job	Run time
build-and-check	2m 6s
test-integration-packages/internal	41s
test-integration-sdk	56s
test-integration-web	40s
test-packages/billing	47s
test-integration-common	39s
test-packages/internal	40s
test-integration-backend	42s
test-integration-packages/billing	41s
test-backend	42s
test-integration-packages/agent-runtime	40s
test-integration-cli	48s
test-integration-npm-app	40s
test-.agents	35s
test-integration-.agents	1m 18s
test-npm-app	47s
test-packages/agent-runtime	53s
test-sdk	52s
test-cli	1m 9s
test-web	44s
test-common	47s
	17m 47s

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