Revert "Added read_ahead_is_registered_, made deregistration happen earlier in ~page_cache_t."

This reverts commit 003143ba1d0c13c8b28331a2260ece41bb896b43.
Revert "Okay, got rid of page_readahead_cb_t, just put the stuff in page_cache_t."

This reverts commit e2fa8e2023b61fe70dd346a29c595f45651cdc21.
Revert "Added page_readahead_cb_t, which does nothing."

This reverts commit a6528fecb4bb8493adb377807d9162dcc14bf4fc.