Removing google drive integration

It's sad but it's not working. For multiple releases this should be
already broken which shows how often it's used.

As there is also a security issue related to that, it's better to
remove the feature completely. Whoever wants to rewrite it, feel free to
go.

This commit removes the Google Drive integration from HackMD's Frontend
editor and this way removes the need to provide any API key and Client
ID in the frontend.

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

README.md

@@ -272,7 +272,7 @@ There are some config settings you need to change in the files below.
 | ------- | --------- | ----------- |
 | facebook, twitter, github, gitlab, mattermost, dropbox, google, ldap, saml | environment variables or `config.json` | for signin |
 | imgur, s3, minio | environment variables or `config.json` | for image upload |
-| google drive(`google/apiKey`, `google/clientID`), dropbox(`dropbox/appKey`) | `config.json` | for export and import |
+| dropbox(`dropbox/appKey`) | `config.json` | for export and import |
 
 ## Third-party integration OAuth callback URLs