Remove version alias for 2.1.0

After the 2.1.0 ruby will not use the patchlevel for 2.1+ releases.

The next release will be 2.1.1 so we don't need these aliases anymore.

See https://github.com/sstephenson/ruby-build/commit/9e76ceb8bf772115e34c98802826da3884dd0b2c and linked tweet.