fsverity: remove debug messages and CONFIG_FS_VERITY_DEBUG

I've gotten very little use out of these debug messages, and I'm not
aware of anyone else having used them.

Indeed, sprinkling pr_debug around is not really a best practice these
days, especially for filesystem code.  Tracepoints are used instead.

Let's just remove these and start from a clean slate.

This change does not affect info, warning, and error messages.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20221215060420.60692-1-ebiggers@kernel.org