Remove page cache

We do not currently use the page cache functionality from Turbolinks in Shopify. While we did have some basic tests covering page caching, it is far from being sufficiently covered against possible edge cases.

@qq99 and I were talking about this today and decided that it's safer to remove page caching from Turbograft for now, and only re-add if the need arises and we are able to dedicate the time to test and validate it.

@tylermercier @pseudomuto @qq99 you guys all cool with this? Any objections?