Remove additional queries looking for cached dimensions

[volksman]

It seems this has been in the package since 2014 but I just noticed it today while trying to optimize server response times. I noticed that when I asked for with width and height of an image I was generating at least 1 extra query per image.

After reviewing the code it looks like a pretty easy win to disable the query to check if there is a dimension cached for a particular image which is being called regardless of the THUMBNAIL_CACHE_DIMENSIONS setting. This seems counter intuitive to me so I've added an if statement to check for the setting before running extra queries to find dimensions.