Remove extra dependency for deoplete-solargraph.py

[uplus]

I separated deoplete-solargraph and solargraph-utils.py from the viewpoint of maintainability and modularity.
This made development very easy, but for users it would be troublesome to add more packages to manage.

But, if solargraph-utils.py is merged, dependencies on solargraph and python neovim-client remain.
So, I don't feel much merit in merging deoplete-solargraph.py.

The best solution I think is to make solargraph-utils.py manageable with vim's plugin manager.

[iovis]

But solargraph is used by other editors for autocompletion and neovim client is used by other plugins in neovim. deoplete-solargraph.py is only used by this plugin, that's why I find it troublesome. But I agree it's probably easier to maintain separately.

[halftan]

No, it doesn't make it easier to maintain this plugin and it's added extra effort to develop/bugfix this plugin since I need to open up two workspaces and switching between each other frequently. The two projects are coupling so close to each other and I highly doubt that there will be another repo depending on solargraph-utils.py, thus there's no benefit on separating these two repos.