remove redundant info (#3539)

* remove redundant info

* fix a few more

Author: mairaw

xml/Microsoft.CSharp/Compiler.xml

@@ -20,9 +20,7 @@
     </Attribute>
   </Attributes>
   <Docs>
-    <summary>**NOTE: This API is now obsolete.** The non-obsolete alternative is <see cref="T:Microsoft.CSharp.CSharpCodeProvider" />.
-
-Managed wrapper for the C# compiler.</summary>
+    <summary>Managed wrapper for the C# compiler.</summary>
     <remarks>
       <format type="text/markdown"><![CDATA[
 

xml/Microsoft.CSharp/CompilerError.xml

@@ -20,7 +20,7 @@
     </Attribute>
   </Attributes>
   <Docs>
-    <summary>**NOTE: This API is now obsolete.** The non-obsolete alternative is <see cref="T:Microsoft.CSharp.CSharpCodeProvider" />. Represents an error or diagnostic message reported by the compiler.</summary>
+    <summary>Represents an error or diagnostic message reported by the compiler.</summary>
     <remarks>To be added.</remarks>
     <related type="Article" href="~/docs/csharp/language-reference/compiler-messages/index.md">C# Compiler Errors and Warnings</related>
   </Docs>

xml/Microsoft.CSharp/ErrorLevel.xml

@@ -19,7 +19,7 @@
     </Attribute>
   </Attributes>
   <Docs>
-    <summary>**NOTE: This API is now obsolete.** The non-obsolete alternative is <see cref="T:System.CodeDom.Compiler.ICodeCompiler" />. Specifies the severity of a compiler diagnostic.</summary>
+    <summary>Specifies the severity of a compiler diagnostic.</summary>
     <remarks>To be added.</remarks>
   </Docs>
   <Members>