[Instrumentation] Remove -pgo-instr-old-cfg-hashing (#77357)

It's been more than 3 years since -pgo-instr-old-cfg-hashing was
introduced by:

  commit 120e66b3
  Author: Hiroshi Yamauchi <yamauchi@google.com>
  Date:   Tue Jul 28 10:09:49 2020 -0700

I don't think anyone really cares about the ability to use the old CFG
hashing at this point.