Remove code for legacyMode

[dafydd-t]

Currently the legacyMode, detected if the args contain certain openssh flags, doesn't work: The logic is never reached because since #59, we parse the flags before we check for legacyMode.

I think we should remove the code for legacyMode, as we don't need it for local dev anymore and there don't seem to be anyone using an unpinned version of the agent with legacyMode as we've not had any complaints about it breaking.