internal/bytealg: unexport {Last,}IndexRabinKarp helpers

internal/bytealg: unexport {Last,}IndexRabinKarp helpers

After CL 538737 these no longer need to be exported. They are only
used in internal/bytealg and can be unexported.

Change-Id: Idd405f397c7ec9f96425d2b7e0e74de61daa7a6e
Reviewed-on: https://go-review.googlesource.com/c/go/+/741920
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>