ci: Remove cache_key calculation

[thompson-tomo]

Currently we are calculating a cache key using

echo “cache_key=mri” >> $GITHUB_OUTPUT
if [[ “$RUBY_INPUT” == “jruby” ]]; then
  echo “cache_key=jruby” >> $GITHUB_OUTPUT
elif [[ “$RUBY_INPUT” == “truffleruby” ]]; then
  echo “cache_key=truffleruby” >> $GITHUB_OUTPUT
fi

However in no scenario is any value other than mri determined due to $RUBY_INPUT being jruby-10.0.2.0 etc and the usage already differentiate based on ruby version.

As such the variable and it's single usage is removed.