Remove unnecessary rule severity settings

The inherited "Warning" severity will result in error reports on CI builds
due to the use of TreatWarningsAsErrors.

src/Compilers/CSharp/CSharpCodeAnalysisRules.ruleset

@@ -5,7 +5,4 @@
   <Rules AnalyzerId="Roslyn.Diagnostics.Analyzers.CSharp" RuleNamespace="Roslyn.Diagnostics.Analyzers.CSharp">
     <Rule Id="RS0013" Action="Warning" />
   </Rules>
-  <Rules AnalyzerId="Roslyn.Diagnostics.Analyzers.CSharp" RuleNamespace="Roslyn.Diagnostics.Analyzers.CSharp">
-    <Rule Id="RS0018" Action="Error" />
-  </Rules>
 </RuleSet>
\ No newline at end of file

src/Compilers/VisualBasic/BasicCodeAnalysisRules.ruleset

@@ -8,7 +8,4 @@
   <Rules AnalyzerId="Roslyn.Diagnostics.Analyzers.VisualBasic" RuleNamespace="Roslyn.Diagnostics.Analyzers.VisualBasic">
     <Rule Id="RS0013" Action="Warning" />
   </Rules>
-  <Rules AnalyzerId="Roslyn.Diagnostics.Analyzers.VisualBasic" RuleNamespace="Roslyn.Diagnostics.Analyzers.VisualBasic">
-    <Rule Id="RS0018" Action="Error" />
-  </Rules>
 </RuleSet>
\ No newline at end of file