Delete a negative-cache test that tests nothing (closes #347)

TestProxy_Get_ReturnsErrorForNegativeCachedURL never called Get; it only
checked ErrNegativeCached against itself. The test above it covers Get
returning ErrNegativeCached. Deletion approved by sneak on the issue.

Model: opus-5-5

internal/imageproxy/negative_cache_internal_test.go

@@ -82,15 +82,3 @@ func TestProxy_Get_ServesCachedVariantOfNegativeCachedURL(t *testing.T) {
 			err, ErrNegativeCached)
 	}
 }
-
-func TestProxy_Get_ReturnsErrorForNegativeCachedURL(t *testing.T) {
-	t.Parallel()
-
-	// This test verifies that Proxy.Get() checks the negative cache
-	// We can't easily test the full pipeline without vips, but we can
-	// verify the error type
-	err := ErrNegativeCached
-	if !errors.Is(err, ErrNegativeCached) {
-		t.Error("ErrNegativeCached should be identifiable with errors.Is")
-	}
-}