Remove unused _decompress_image_file from powervm operator class

While looking at bug 1214985 I noticed that the _decompress_image_file
method in the BaseOperator class wasn't being used, so this patch simply
cleans that up to avoid future confusion.

Related-Bug: #1214985

Change-Id: Ic6f2010727dcc1dfabc310ffa3bd5ca0b6799537
Merge "Remove unused _decompress_image_file from powervm operator class"