remove the gist module from sugar (#120)

should
- close #106 
 
# description
as `nu-git-manager-sugar gist` hasn't been touched since #26, it's old,
unmaintained, possibly broken and does not fit the _philosophy_ of NGM,
e.g. it does not export `gm ...` commands 🤔

this PR removes the `gist` module from `sugar` for now, until someone
can reimplement it and make sure it's functional 👍