DebugTools: remove unused declaration in a test.

THis one is split into imageFileZeroDelta() and imageFileNonZeroDelta().

src/Magnum/DebugTools/Test/CompareImageTest.cpp

@@ -89,7 +89,6 @@ struct CompareImageTest: TestSuite::Tester {
     void imageNonZeroDelta();
     void imageNonZeroDeltaNoPixels();
     void imageError();
-    void imageFile();
     void imageFileZeroDelta();
     void imageFileNonZeroDelta();
     void imageFileError();