Remove code snippets

[nicolasstucki]

After discussions at a Scala Center meeting, the following conclusion was reached the conclusion

• snippets extension should be removed, leave it to community to maintain. It's OK to install multiple snippet extensions so it's fine to have multiple competing community extensions.
• syntax rules stays in the repo because it's not possible to have a multiple Scala syntax extensions.

Originally posted by @nicolasstucki in #17 (comment)

[olafurpg]

I haven't unpublished the snippet extension yet. I consulted with @gabro how to best handle this situation and he recommended to publish a new update

• changing the name to mention the package is deprecated
• updating the readme explaining that this package is no longer maintained and users are encouraged to publish their own snippet extension.

wdyt @nicolasstucki ?