cleanup: remove legacy backend and npm-app #5074

Job	Run time
build-and-check	2m 2s
test-integration-sdk	58s
test-integration-packages/agent-runtime	37s
test-integration-common	46s
test-integration-npm-app	53s
test-cli	1m 9s
test-integration-cli	39s
test-integration-packages/billing	44s
test-packages/agent-runtime	58s
test-integration-web	44s
test-packages/billing	53s
test-integration-.agents	1m 15s
test-web	42s
test-sdk	53s
test-backend	40s
test-.agents	48s
test-packages/internal	40s
test-integration-backend	47s
test-integration-packages/internal	47s
test-npm-app	40s
test-common	49s
	18m 24s

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