remove mention of custom metrics for agent check (#7041)
remove mention of custom metrics for agent check (#7041)

agent metrics check doesn't have any custom metrics: https://datadoghq.atlassian.net/browse/DOCS-496
DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core,DataDog/integrations-core