This PR changes how our own class derives from and replaces the Chromium version of `RenderViewContextMenu`, to be more in line of how this is done for other classes. This change does away with `BraveRenderViewContextMenu`, and makes it the defacto `RenderViewContextMenu`, rather than `RenderViewContextMenu` being a define to the Brave class. This means that with this change there's now only `RenderViewContextMenu` and `RenderViewContextMenu_ChromiumImpl`. Resolves https://github.com/brave/brave-browser/issues/54023