Remove duplicate top-level required on credential definition (Azure#58408)

Both Azure.Core and System.ClientModel ConfigurationSchema.json segments
declared `required: ["CredentialSource"]` at the top of
`definitions/credential`. When the appsettings schema combiner merges
segments by definition name, it concatenates `required` arrays without
dedup, producing `["CredentialSource", "CredentialSource"]` in the
consumer's combined AppSettingsSchema.json. System.ClientModel already
requires CredentialSource, so removing it from Azure.Core's top-level
credential definition eliminates the duplicate while preserving all
`if`-clause requireds and the chainableCredential definition.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: m-nash

sdk/core/Azure.Core/schema/ConfigurationSchema.json

@@ -121,9 +121,6 @@
           ]
         }
       },
-      "required": [
-        "CredentialSource"
-      ],
       "allOf": [
         {
           "if": {