Follow-up on 12565daf88 PR comment

- remove `explicitValue` variables from two `DefaultHtmlGenerator` methods

This follows up a [code review comment](https://github.com/aspnet/Mvc/pull/1834/files#r23120381)
related to `DefaultHtmlGenerator.GenerateCheckBox()`.  Same issue appears in
`DefaultHtmlGenerator.GenerateRadioButton()`.

@pranavkm -
> Also a different variable name? There's a parameter isExplicitValue that
> follows which always has a false value. Reusing this name seems messy.

My response to that comment should not be repeated.  @pranavkm was correct
that the only issue here is naming.  `explicitValue` was too easily
confused with the `isExplicitValue` argument used a few lines later.
They had separate purposes: `explicitValue` related to the "checked"
attribute while `isExplicitValue` relates to the "value" attribute.

src/Microsoft.AspNet.Mvc.Core/Rendering/Html/DefaultHtmlGenerator.cs

@@ -110,10 +110,9 @@ namespace Microsoft.AspNet.Mvc.Rendering
             }
 
             var htmlAttributeDictionary = GetHtmlAttributeDictionaryOrNull(htmlAttributes);
-            var explicitValue = isChecked.HasValue;
-            if (explicitValue && htmlAttributeDictionary != null)
+            if (isChecked.HasValue && htmlAttributeDictionary != null)
             {
-                // Explicit value must override dictionary
+                // Explicit isChecked value must override "checked" in dictionary.
                 htmlAttributeDictionary.Remove("checked");
             }
 
@@ -124,7 +123,7 @@ namespace Microsoft.AspNet.Mvc.Rendering
                 metadata,
                 name,
                 value: "true",
-                useViewData: (metadata == null && !explicitValue),
+                useViewData: (metadata == null && !isChecked.HasValue),
                 isChecked: isChecked ?? false,
                 setId: true,
                 isExplicitValue: false,
@@ -325,10 +324,9 @@ namespace Microsoft.AspNet.Mvc.Rendering
                     string.Equals(model.ToString(), valueString, StringComparison.OrdinalIgnoreCase);
             }
 
-            var explicitValue = isChecked.HasValue;
-            if (explicitValue && htmlAttributeDictionary != null)
+            if (isChecked.HasValue && htmlAttributeDictionary != null)
             {
-                // Explicit value must override dictionary
+                // Explicit isChecked value must override "checked" in dictionary.
                 htmlAttributeDictionary.Remove("checked");
             }