cleanup: remove legacy backend and npm-app #5058

Job	Run time
build-and-check	1m 56s
test-integration-npm-app	42s
test-integration-packages/billing	39s
test-integration-backend	42s
test-integration-packages/internal	41s
test-integration-sdk	56s
test-integration-web	41s
test-integration-packages/agent-runtime	44s
test-integration-.agents	1m 13s
test-backend	38s
test-.agents	45s
test-cli	1m 8s
test-integration-cli	34s
test-integration-common	40s
test-common	38s
test-npm-app	39s
test-packages/agent-runtime	52s
test-packages/internal	42s
test-packages/billing	41s
test-web	49s
test-sdk	48s
	17m 8s

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