Remove unneeded casts of BufferGetPage() result

Remove unneeded casts of BufferGetPage() result

BufferGetPage() already returns type Page, so casting it to Page
doesn't achieve anything.  A sizable number of call sites does this
casting; remove that.

This was already done inconsistently in the code in the first import
in 1996 (but didn't exist in the pre-1995 code), and it was then
apparently just copied around.

Author: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://www.postgresql.org/message-id/flat/CALdSSPgFhc5=vLqHdk-zCcnztC0zEY3EU_Q6a9vPEaw7FkE9Vw@mail.gmail.com