[tools] Remove some dead code in the class redirector rewriter. (#18695)

This also fixes a compiler warnings:

> tools/common/Rewriter.cs(333,25): warning CS8604: Possible null
reference argument for parameter 'path1' in 'string Path.Combine(string
path1, string path2)'.

And make nullability warnings in dotnet-linker show up as errors to
prevent future introduction of nullability warnings.

tools/common/Rewriter.cs

@@ -20,7 +20,6 @@ namespace ClassRedirector {
 		const string classPtrName = "class_ptr";
 		CSToObjCMap map;
 		string pathToXamarinAssembly;
-		string? outputDirectory = null;
 		Dictionary<string, FieldDefinition> csTypeToFieldDef = new Dictionary<string, FieldDefinition> ();
 		IEnumerable<AssemblyDefinition> assemblies;
 		AssemblyDefinition xamarinAssembly;
@@ -328,11 +327,6 @@ namespace ClassRedirector {
 			}
 		}
 
-		string ToOutputFileName (string pathToInputFileName)
-		{
-			return Path.Combine (outputDirectory, Path.GetFileName (pathToInputFileName));
-		}
-
 		void MarkForSave (AssemblyDefinition assembly)
 		{
 			var annotations = linkContext.Annotations;

tools/dotnet-linker/dotnet-linker.csproj

@@ -4,6 +4,7 @@
     <RootNamespace>dotnet_linker</RootNamespace>
     <DefineConstants>$(DefineConstants);BUNDLER</DefineConstants>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    <WarningsAsErrors>nullable</WarningsAsErrors>
   </PropertyGroup>
 
   <Import Project="..\..\eng\Versions.props" />