[webkit-efl] Remove pre-render priority flags

[JungJik Lee]

Hello, webkit-efl port developer!

I am slowly preparing pre-rendering code to sync with upstream code.
Before I starts in earnest, I try to remove pre-render priority code.
Pre-rendering concept is for drawing tiles outside viewport in idle time.
So it always works after drawing the tiles in viewport.
But there is a priority flags (low,high) to add a request in drawing queue.
I think it is not necessary for the present, might be useful in future.
In upstream code, there is no priority flags for pre-rendering,
I try to remove _Ewk_Tiled_Backing_Store_Pre_Render_Priority to keep pace with upstream.
https://bugs.webkit.org/show_bug.cgi?id=65504

Thank you for reading!
JungJik Lee