Removed legacy PropertyConfiguration class

With the `ItemConfiguration` being added as a property to `ItemMetadata` (74cb817), and the mapping services being updated to rely upon `ItemMetadata` to get their configuration data (2f482b1, fe39302), there is no longer a need for the legacy `PropertyConfiguration` class, or to differentiate between item and property differentiation more broadly.

Indeed, the main benefit of this was to allow strongly typed access to the `MemberAccessor`, instead of general access to `ItemMetadata`. Since the order if flipped, with `ItemConfiguration` being retrieved _via_ `ItemMetadata` (or `MemberAccessor`), there is always strongly typed access to these objects independent of the configuration object.

Author: JeremyCaney

OnTopic/Mapping/Internal/ItemConfiguration.cs

@@ -31,10 +31,6 @@ namespace OnTopic.Mapping.Internal {
   ///     then the <see cref="ITopicMappingService"/> will instead use the value defined by that attribute, thus allowing a
   ///     property on the DTO to be aliased to a different property or attribute name on the source <see cref="Topic"/>.
   ///   </para>
-  ///   <para>
-  ///     The <see cref="ItemConfiguration"/> works with both <see cref="ParameterInfo"/> and <see cref="PropertyInfo"/>
-  ///     instances, whereas the <see cref="PropertyConfiguration"/> works exclusively with <see cref="PropertyInfo"/> instances.
-  ///   </para>
   /// </remarks>
   internal class ItemConfiguration {