cleanup: remove legacy backend and npm-app #5068

Job	Run time
build-and-check	1m 57s
test-backend	40s
test-packages/agent-runtime	56s
test-npm-app	38s
test-common	45s
test-packages/billing	45s
test-.agents	38s
test-sdk	49s
test-cli	1m 13s
test-integration-.agents	1m 12s
test-integration-backend	44s
test-packages/internal	41s
test-web	47s
test-integration-sdk	53s
test-integration-packages/agent-runtime	38s
test-integration-cli	41s
test-integration-packages/internal	46s
test-integration-npm-app	42s
test-integration-packages/billing	40s
test-integration-web	37s
test-integration-common	42s
	17m 24s

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