Remove Google+ social media from footer

[A. Mazzoccone]

Google + was shutdown 🤙.

Changes should be done in files:
https://github.com/particl/particl.io/blob/master/_includes/social-icons.html#L68
https://github.com/particl/particl.io/blob/master/_sass/layout/_navigation.scss#L199

I could make the PR if accepted 👍.
Thanks for let us contribute!

[Martin Allien]

Google+ link removed already, thanks!