[SPARK-15879][DOCS][UI] Update logo in UI and docs to add "Apache"

## What changes were proposed in this pull request?

Use new Spark logo including "Apache" (now, with crushed PNGs). Remove old unreferenced logo files.

## How was this patch tested?

Manual check of generated HTML site and Spark UI. I searched for references to the deleted files to make sure they were not used.

Author: Sean Owen <sowen@cloudera.com>

Closes #13609 from srowen/SPARK-15879.