Remove unused code from CompressJob

Update comments that didn't make sense for this class.
Remove the unused processDocument() method.
Simplify the toString() method, which is only used in log messages.

Bug: 439976731
Flag: EXEMPT no functional change
Test: atest DocumentsUIGoogleTests:com.android.documentsui.services
Change-Id: Iaaf6da4b22a3ec161bdc6e161374c97b1d3760b2