cleanup: remove legacy backend and npm-app #5071

Job	Run time
build-and-check	2m 4s
test-integration-packages/agent-runtime	41s
test-integration-packages/billing	46s
test-integration-.agents	1m 7s
test-integration-npm-app	40s
test-integration-common	40s
test-integration-sdk	50s
test-integration-backend	42s
test-integration-web	47s
test-npm-app	46s
test-integration-cli	47s
test-.agents	40s
test-backend	39s
test-common	41s
test-packages/internal	40s
test-web	48s
test-integration-packages/internal	44s
test-sdk	51s
test-cli	1m 20s
test-packages/agent-runtime	59s
test-packages/billing	43s
	17m 55s

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