The amendment is basically structured like this:
Because A is true, right B must not be infringed.
If "A" is no longer true, then B no longer applies. I don't have to repeal anything, all I have to do is demonstrate that a militia isn't necessary for the security of the state.
Also, I've got to laugh at the "State" being the Redcoats. You do realize that we had kicked out the Redcoats by 1791, right?