Remove mentions of Fields on ImportAttribute (#4422)

ImportAttribute has never supported fields.  It looks like the docs were wrong.

Author: ericstj

xml/System.Composition/ImportAttribute.xml

@@ -24,7 +24,7 @@
     </Attribute>
   </Attributes>
   <Docs>
-    <summary>Specifies that a property, field, or parameter value should be provided by the host container.</summary>
+    <summary>Specifies that a property or parameter value should be provided by the host container.</summary>
     <remarks>
     </remarks>
   </Docs>
@@ -100,9 +100,9 @@
         <ReturnType>System.Boolean</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets or sets a value that indicates whether the property, field, or parameter will be set to its type's default value when an export with the contract name is not present in the container.</summary>
+        <summary>Gets or sets a value that indicates whether the property or parameter will be set to its type's default value when an export with the contract name is not present in the container.</summary>
         <value>
-          <see langword="true" /> if the property, field, or parameter will be set to its type's default value when there is no export with the contract name in the host container; otherwise, <see langword="false" />. The default is <see langword="false" />.</value>
+          <see langword="true" /> if the property or parameter will be set to its type's default value when there is no export with the contract name in the host container; otherwise, <see langword="false" />. The default is <see langword="false" />.</value>
         <remarks>
         </remarks>
       </Docs>