[dev.fuzz] Revert "[dev.fuzz] testing: convert seed corpus values where possible"

This reverts commit 413c125d.

Reason for revert: Giving this more thought, we've decided that
converting types under the hood may cause unexpected behavior to
users. This is a feature that can always be added after more
consideration has been done, but is not something that can be
removed due to the backwards compatibility promise.

Updates golang/go#45593

Change-Id: I79bab24979d7e4c294e6cb6455d4c7729d6a0efb
Reviewed-on: https://go-review.googlesource.com/c/go/+/350251


Trust: Katie Hockman <katie@golang.org>
Trust: Joe Tsai <joetsai@digital-static.net>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Joe Tsai <joetsai@digital-static.net>