[Java.Runtime.Environment] Remove decl for java_interop_strdup (#1256)

It's not actually used within `Java.Runtime.Environment.dll`.

Author: filipnavara

src/Java.Runtime.Environment/Java.Interop/MonoRuntimeValueManager.cs

@@ -407,9 +407,6 @@ partial class JreNativeMethods {
 		[DllImport (JavaInteropLib, CallingConvention=CallingConvention.Cdecl)]
 		internal static extern int java_interop_gc_bridge_remove_current_app_domain (IntPtr bridge);
 
-		[DllImport (JavaInteropLib, CallingConvention=CallingConvention.Cdecl)]
-		internal static extern IntPtr java_interop_strdup (string value);
-
 		[DllImport (JavaInteropLib, CallingConvention=CallingConvention.Cdecl)]
 		internal static extern int java_interop_gc_bridge_set_bridge_processing_field (IntPtr bridge, RuntimeTypeHandle type_handle, string field_name);