Revert "Added migration notification for rails plugin"

This reverts commit 1493d88e, made
to put a deprecated notice on rails3 and rails4 plugins, so that
users would migrate to the unified rails plugin.

It's been 1.5 years since that, so everyone should be migrated to
the good one.

plugins/rails3/rails3.plugin.zsh

-echo "It looks like you have been using the 'rails3' plugin,"
-echo "which has been deprecated in favor of a newly consolidated 'rails' plugin."
-echo "You will want to modify your ~/.zshrc configuration to begin using it."
-echo "Learn more at https://github.com/robbyrussell/oh-my-zsh/pull/2240"

plugins/rails4/rails4.plugin.zsh

-echo "It looks like you have been using the 'rails4' plugin,"
-echo "which has been deprecated in favor of a newly consolidated 'rails' plugin."
-echo "You will want to modify your ~/.zshrc configuration to begin using it."
-echo "Learn more at https://github.com/robbyrussell/oh-my-zsh/pull/2240"