Remove the redshiftAsyncQuerySupport feature toggle + styling improvements

[idastambuk]

Same as grafana/athena-datasource#316

Did a few things here:

• remove the redshiftAsyncQuerySupport feature toggle
• removed the use of deprecated width-30 class to instead use Grafana theme spacing
• fixed the Run query button to not be disabled if there is no change in the editor. I was kinda bothered by this cause I sometimes just wanted to rerun the same query and not have to go all the way up to the Grafana dashboard refresh button. Also I realized this is how Cloudwatch header buttons work - the Run button is never disabled

I thought the changes were small enough to squeeze in here, but lmk if you'd rather I separate them into 2 or 3 PRs.