Remove username and password env vars from README (Azure#48626)

Author: scottaddie

sdk/identity/Azure.Identity/README.md

@@ -227,15 +227,6 @@ Not all credentials require this configuration. Credentials that authenticate th
 |`AZURE_CLIENT_CERTIFICATE_PASSWORD`|(optional) the password protecting the certificate file (currently only supported for PFX (PKCS12) certificates)
 |`AZURE_CLIENT_SEND_CERTIFICATE_CHAIN`|(optional) send certificate chain in x5c header to support subject name / issuer based authentication
 
-### Username and password
-
-|Variable name|Value
-|-|-
-|`AZURE_CLIENT_ID`|ID of a Microsoft Entra application
-|`AZURE_TENANT_ID`|ID of the application's Microsoft Entra tenant
-|`AZURE_USERNAME`|a username (usually an email address)
-|`AZURE_PASSWORD`|that user's password
-
 ### Workload identity (`DefaultAzureCredential`)
 
 |Variable name|Value