fil_page_type_validate(): Remove. This debug check was mostly redundant
and added little value to the code paths that deal with page_compressed
or encrypted pages.

fil_get_page_type_name(): Remove; unused function.

fil_space_decrypt(): Return an error if the page is not
supposed to be encrypted. It is possible that an unencrypted page
contains a nonzero key_version field even though it is not supposed
to be encrypted. Previously we would crash in such a situation.

buf_page_decrypt_after_read(): Simplify the code. Remove some
unnecessary error message about temporary tablespace corruption.
This is where we would usually invoke fil_space_decrypt().