Remove deprecated folders

[tlgimenes]

Hi people!

We are currently developing new components to store-ui while still maintaining the old components inside a folder called "deprecated". Since we are using tsdx for bundling, we can't really create two entrypoints and the bundle analyser analyses all components, the new and the old ones.
Also, last time I tried creating a store using only the new components I had an issue where theme-ui was being included into my final bundle.
Another problem this migration causes is that we can't really advance really fast because we also need to maintain some old APIs and packages, like theme-store and plugin-theme-ui. For this and other problems, I propose:

1. Create a branch called legacy with the current state of the master branch
2. Remove the following packages:
   2.1. gatsby-plugin-theme-store
   2.2. gatsby-plugin-theme-ui
   2.3. gatsby-plugin-i18n
   2.4. gatsby-plugin-performance
   2.5. deprecated folder inside store-ui
   2.6. decrease max bundle size for store-ui to 15Kb

With this, I hope to develop the feature faster without having to backport everything everywhere