Drop vestigal --agents ssh keychain option

The default is SSH only anyway, and if the default changes to GPG too,
that's fine and desired.

.zshrc

@@ -4,7 +4,7 @@ export LANG=en_US.UTF-8
 if [ -n "$SSH_CLIENT" ] || [ -n "$SSH_TTY" ]; then
     SESSION_TYPE=ssh
 else
-    eval `keychain --eval --agents ssh id_rsa`
+    eval `keychain --eval id_rsa`
 fi
 
 # Enable Powerlevel10k instant prompt. Should stay close to the top of ~/.zshrc.