Remove RLangMode.zip from repo

[jeremydouglass]

Currently a large release file docs/RLangMode.zip is checked in as part of the repo -- it started being checked in here.

4643a9e

This isn't actually necessary for releases -- they are uploaded separately. Checking in every release as a 32MB zip makes the repo large over time. Consider removing, adding it to .gitignore -- or using tools to do a repo rewrite that removes it while preserving all history.

A problem, once a file is checked in, I believe (?) that .gitignore won't prevent future updates to that file.

[jeremydouglass]

In the meantime, we could remove the zip and recommend a shallow clone for those who need it:

git clone https://github.com/processing-r/Processing.R.git --depth 1