vdl: Remove rpc.BindOpt from generated code.

While cleaning up the Glossary on our website I realized
that "BindOpt", an old relic that makes no sense in the *context.T
world, is still lying around.

This commit removes usage of BindOpt from VDL generated code.
Once this (and related commits) is in, I intend to remove the BindOpt
interface from v.io/v23/rpc.

MultiPart: 1/4
Change-Id: I1933e0ea8b4793a97f71722c73b7b90e599af3ce
20 files changed