fix: BindingExpression failed to apply string explicit source.

src/Uno.UI.Tests/BinderTests/Given_Binder.cs

@@ -763,6 +763,23 @@ public void When_Source_Complex()
 			Assert.AreEqual(42, SUT.Tag);
 		}
 
+		[TestMethod]
+		public void When_Source_String()
+		{
+			var source = "Test";
+			var SUT = new Windows.UI.Xaml.Controls.Grid();
+
+			SUT.SetBinding(
+				Windows.UI.Xaml.Controls.Grid.TagProperty,
+				new Binding()
+				{
+					Source = source
+				}
+			);
+
+			Assert.AreEqual(source, SUT.Tag);
+		}
+
 		[TestMethod]
 		public void When_Subject_Source_Complex()
 		{

src/Uno.UI/DataBinding/BindingExpression.cs

@@ -347,13 +347,6 @@ private void ApplyExplicitSource()
 					this.Log().DebugFormat("Applying explicit source {0} on {1}", ExplicitSource?.GetType(), _view.Target?.GetType());
 				}
 
-				var resourceName = ExplicitSource as string;
-
-				if (resourceName.HasValue())
-				{
-					_dataContext = Uno.UI.DataBinding.WeakReferencePool.RentWeakReference(this, ResourceHelper.FindResource(resourceName));
-				}
-
 				ApplyBinding();
 			}
 		}