Remove `credentials.Bundle` from `NewSubConnOptions`

[dfawley]

This is potentially a security issue in that it allows a balancer to inject credentials.

This was added for grpclb to be able to set the "mode" of the user's credentials (e.g. ComputeEngineCreds). Instead, grpclb should set that same mode string in the Attributes for the Addresses, and the credentials should retrieve the mode and adjust accordingly.

In this regard, mode switching can be removed from credentials.Bundle, but the ability to bundle call+transport creds into a single object is still beneficial.