Revert "ASTGen: add a workaround for CSC conformance visibility"

This reverts commit 5d0c5a6.

Author: rintaro

lib/ASTGen/Sources/ASTGen/Macros.swift

@@ -127,11 +127,7 @@ enum ASTGenMacroDiagnostic: DiagnosticMessage, FixItMessage {
   var message: String {
     switch self {
     case .thrownError(let error):
-      if let err = error as? PluginError {
-        return err.description
-      } else {
-        return String(describing: error)
-      }
+      return String(describing: error)
 
     case .oldStyleExternalMacro:
       return "external macro definitions are now written using #externalMacro"