Toggle the authenticate capability

[Roy A. Gilmore]

Hi,

I have a keypair that was initially generated with the defaults, so the
signing key also has the authenticate capability enabled. I want to add
a separate authentication subkey for use with an OpenPGP smartcard. Is
there any way to turn the authenticate capability off on the signing
key? It doesn't sound like it should be that difficult, but I've
searched using several different search terms, and I can't seem to find
a way to do this.

Roy A. Gilmore