Update INSTALL.md

INSTALL.md

@@ -231,7 +231,6 @@ vim /srv/walnut/var/$my_domain/config.json
 ```json
 { "mailgun.org": {
     "apiKey": "key-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
-  , "apiPublicKey": "pubkey-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
   , "auth": {
       "user": "robtherobot@example.com"
     , "pass": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"