Remove unnecessary imports in UIImageView & UIWebView+AFNetworking headers

This change brings the two headers mentioned inline with the other UIKit+AFN categories that do not import other AFN headers.

The imports are redundant if you are including `AFNetworking.h`. If you are *not* importing AFNetworking.h and wish to use the protocols and classes that are now forward declared, you'd need to import the particular headers yourself.