The /opt/aws/amitools directory contains the Amazon AWS EC2 command line utitlies for working with Amazon Machine Images (AMIs). This package is installed by default on the Amazon AMIs, but is dependent on Ruby. By uninstalling Ruby, you uninstalled anything that depended on it as well, including this package (aws-amitools-ec2.noarch). You can't install this package back using YUM as it will re-install the original Ruby package.
You can download the AMI Tools package from here and manually extract them into the /opt/aws/amitools directory. In my installation the tools are stored in that directory with the name ec2-VERSION, and there is a symlink called ec2 that points to this folder.
You may also want the API Tools from here as well, which would go into the /opt/aws/apitools directory and would have been removed by uninstalling Ruby as well.