Remove aggregator.theme.css deprecated by entity conversion #293318

Problem/Motivation

The CSS file in the aggregator module does not apply to any actual HTML provided by the module's markup. This happened after the refactoring in #293318: Convert Aggregator feeds into entities

The remaining CSS is pretty pathetic too.

.aggregator .feed-icon {
  float: right; /* LTR */
  display: block;
}

In the rest of Drupal the .feed-icon doesn't have any special styling. Why does aggregator module need to do something special anyway?

Proposed resolution

Let's just delete the entire file (and its RTL companion.)

Remaining tasks

After this issue's completion, we still have all the other steps outlined at #1921610: [Meta] Architect our CSS.

User interface changes

none

API changes

none