Remove indexer rule.

SixLabors.ruleset

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <RuleSet Name="SixLabors" ToolsVersion="16.0">
   <Rules AnalyzerId="Microsoft.CodeAnalysis.CSharp.Features" RuleNamespace="Microsoft.CodeAnalysis.CSharp.Features">
+    <Rule Id="IDE0056" Action="None" />
     <Rule Id="IDE0057" Action="None" />
   </Rules>
   <Rules AnalyzerId="Roslynator.CSharp.Analyzers" RuleNamespace="Roslynator.CSharp">