Update "Telling Git about your X.509 key" to not assume that GPG and X.509 are the only signing options

GitHub has recently introduced a third way to sign your commits
alongside GPG and X.509: using an SSH key.

At the moment, our copy about X.509 mentions using it "instead of
GPG", which made sense when the only options were GPG or it, but
now it's a bit confusing. This removes that copy.

Author: timrogers

data/reusables/gpg/x-509-key.md

@@ -1,7 +1,7 @@
 
 ## Telling Git about your X.509 key
 
-You can use [smimesign](https://github.com/github/smimesign) to sign commits and tags using S/MIME instead of GPG.
+You can use [smimesign](https://github.com/github/smimesign) to sign commits and tags using S/MIME.
 
 {% data reusables.gpg.smime-git-version %}