Test: remove unused enum from Image tests.

src/Magnum/Test/ImageTest.cpp

@@ -180,8 +180,6 @@ namespace Vk {
         #endif
         return 12;
     }
-
-    enum class CompressedPixelFormat { Bc1SRGBAlpha = 42 };
 }
 
 void ImageTest::constructGeneric() {

src/Magnum/Test/ImageViewTest.cpp

@@ -174,8 +174,6 @@ namespace Vk {
         #endif
         return 12;
     }
-
-    enum class CompressedPixelFormat { Bc1SRGBAlpha = 42 };
 }
 
 template<class T> void ImageViewTest::constructGeneric() {

src/Magnum/Trade/Test/ImageDataTest.cpp

@@ -208,8 +208,6 @@ namespace Vk {
         #endif
         return 12;
     }
-
-    enum class CompressedPixelFormat { Bc1SRGBAlpha = 42 };
 }
 
 void ImageDataTest::constructGeneric() {