Remove redundant static subscripts on attributes (#44)

Some properties in generated code have a layer of indirection due to their use of `ReadonlyAttribute` and `ReadWriteAttribute`. There's little value in keeping these static subscripts around and introducing the indirection. Additionally, it makes makes some issues on property code hard to debug when attempting to change union types generation logic in my investigation of #43.

Sources/ECMAScript/Attributes.swift

@@ -10,13 +10,8 @@ import JavaScriptKit
     }
 
     @inlinable public var wrappedValue: Wrapped {
-        get { ReadWriteAttribute[name, in: jsObject] }
-        nonmutating set { ReadWriteAttribute[name, in: jsObject] = newValue }
-    }
-
-    @inlinable public static subscript(name: JSString, in jsObject: JSObject) -> Wrapped {
         get { jsObject[name].fromJSValue()! }
-        set { jsObject[name] = newValue.jsValue }
+        nonmutating set { jsObject[name] = newValue.jsValue }
     }
 }
 
@@ -30,10 +25,6 @@ import JavaScriptKit
     }
 
     @inlinable public var wrappedValue: Wrapped {
-        ReadonlyAttribute[name, in: jsObject]
-    }
-
-    @inlinable public static subscript(name: JSString, in jsObject: JSObject) -> Wrapped {
         jsObject[name].fromJSValue()!
     }
 }