cmd/compile: remove work-arounds for handling underscores in numbers

With math/big supporting underscores directly, there is no need to
manually remove them before calling the math/big conversion routines.

Updates #28493.

Change-Id: I6f865c8f87c3469ffd6c33f960ed540135055226
Reviewed-on: https://go-review.googlesource.com/c/go/+/166417


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>