Revert "renderer-native: Actually use shadow fb when using software rendering"

It was not the lack of forcing the shadow fb that caused slowness, but rather due to the method the shadow fb content was copied onto the scanout fb. With 'clutter: Use cogl_blit_framebuffer() for shadow FB' we'll use a path that shouldn't be slow when copying onto the scanout fb.

Also 437f6b3d accidentally enabled shadow fb when using hw accelerated contexts, due to the cap being set to 1 in majority of drivers. While the kernel documentation for the related field says "hint to userspace to prefer shadow-fb rendering", the name of the hint when exposed to userspace is DRM_CAP_DUMB_PREFER_SHADOW, thus should only be taken into consideration for dumb buffers, not rendering in general.

This reverts commit 437f6b3d.