Add RemoveFlag() function

[tiegz]

This adds the ability to remove a Flag from a FlagSet, e.g. flagSet.RemoveFlag("my-flag")

My end-goal is to allow cobra commands to opt-out of PersistentFlags() that they inherit. Please LMK if I'm on the wrong track here.

(plus a few formatting changes)

[CLAassistant]

All committers have signed the CLA.

[tiegz]

ping

[tiegz]

ping