cpu: Remove default argument values for the update() method in bpreds.

cpu: Remove default argument values for the update() method in bpreds.

These defaults are never used. There was an assert in the predictors
until recently which was asserting that one of the arguments didn't
have the default value, I think to verify that the default wasn't used
by accident(?), but it could be used purposefully. That would cause
gem5 to crash and has been removed.

Beyond that, there's no reason to have default values for those
arguments in the first place, so this change removes them. That makes
the code slightly simpler, and avoids them being used by accident.

Additionally, the defalt values of the arguments made the function
signatures inconsistent, even though they were supposed to override
each other.

JIRA: https://gem5.atlassian.net/browse/GEM5-483

Change-Id: I28f8d2048985c12ec9cac018a868a32bfa20dc6c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30375
Reviewed-by: Hsuan Hsu <hsuan.hsu@mediatek.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>