cleanup: remove legacy backend and npm-app #5066

Job	Run time
build-and-check	1m 59s
test-integration-.agents	1m 11s
test-integration-backend	40s
test-integration-packages/agent-runtime	43s
test-integration-common	40s
test-integration-packages/billing	40s
test-integration-npm-app	44s
test-integration-cli	41s
test-integration-sdk	55s
test-cli	1m 3s
test-integration-web	46s
test-backend	40s
test-.agents	42s
test-integration-packages/internal	46s
test-packages/internal	46s
test-packages/agent-runtime	50s
test-common	39s
test-sdk	53s
test-packages/billing	41s
test-npm-app	47s
test-web	51s
	17m 37s

Provide feedback