Remove :warning where severity isn't supported

Severity isn't supported for csharp_space_after_keywords_in_control_flow_statements.

"Specifying a severity is not supported here and it won't be supported in VS 2019 either.
Instead you will be able to set a severity for formatting violations in general, no matter
which option they come from."

Author: jcansdale

.editorconfig

@@ -69,4 +69,4 @@ csharp_prefer_simple_default_expression = true:suggestion
 csharp_style_inlined_variable_declaration = true:suggestion
 
 # Spacing options
-csharp_space_after_keywords_in_control_flow_statements = true:warning
+csharp_space_after_keywords_in_control_flow_statements = true