C#: Address review comments.

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/frameworks/Format.qll

@@ -233,15 +233,28 @@ class InvalidFormatString extends StringLiteral {
   }
 }
 
+/**
+ * A method call to a method that parses a format string, for example a call
+ * to `string.Format()`.
+ */
+abstract private class FormatStringParseCallImpl extends MethodCall {
+  /**
+   * Gets the expression used as the format string.
+   */
+  abstract Expr getFormatExpr();
+}
+
+final class FormatStringParseCall = FormatStringParseCallImpl;
+
 /**
  * A method call to a method that formats a string, for example a call
  * to `string.Format()`.
  */
-class FormatCall extends MethodCall {
+class FormatCall extends FormatStringParseCallImpl {
   FormatCall() { this.getTarget() instanceof FormatMethod }
 
   /** Gets the expression used as the format string. */
-  Expr getFormatExpr() { result = this.getArgument(this.getFormatArgument()) }
+  override Expr getFormatExpr() { result = this.getArgument(this.getFormatArgument()) }
 
   /** Gets the argument number containing the format string. */
   int getFormatArgument() { result = this.getTarget().(FormatMethod).getFormatArgument() }
@@ -290,23 +303,6 @@ class FormatCall extends MethodCall {
   }
 }
 
-/**
- * A method call to a method that parses a format string, for example a call
- * to `string.Format()`.
- */
-abstract private class FormatStringParseCallImpl extends MethodCall {
-  /**
-   * Gets the expression used as the format string.
-   */
-  abstract Expr getFormatExpr();
-}
-
-final class FormatStringParseCall = FormatStringParseCallImpl;
-
-private class OrdinaryFormatCall extends FormatStringParseCallImpl instanceof FormatCall {
-  override Expr getFormatExpr() { result = FormatCall.super.getFormatExpr() }
-}
-
 /**
  * A method call to `System.Text.CompositeFormat.Parse`.
  */