[NUI] Fix build warnings: delete obsolete warnings

[NUI] Fix build warnings: delete obsolete warnings

warnings : CS0618
  - https://docs.microsoft.com/ko-kr/dotnet/csharp/language-reference/compiler-messages/cs0618

Change 'Obsolete' keyword to 'EditorBrowsable'
If you have classes or methods that you don't want to open to public developers,
you can hide them from the API reference and IntelliSense using the EditorBrowsableAttribute with EditorBrowsableState.Never.
  ->guide : https://msdn.microsoft.com/ko-kr/library/system.componentmodel.editorbrowsableattribute(v=vs.110).aspx

Change-Id: I3ae939c83240b5a119fb0aa1724627068c8c308a
Signed-off-by: huiyu,eun <huiyu.eun@samsung.com>