Remove UIImageView+Letters from bridging headers

Signed-off-by: Marcel Müller <marcel-mueller@gmx.de>

NextcloudTalk/NextcloudTalk-Bridging-Header.h

@@ -60,7 +60,6 @@
 #import "TOActivityCroppedImageProvider.h"
 #import "UIImageView+AFNetworking.h"
 #import "UIImage+CropRotate.h"
-#import "UIImageView+Letters.h"
 #import "MaterialActivityIndicator.h"
 
 #endif /* NextcloudTalk_Bridging_Header_h */