remove unused image asset

// FREEBIE

Signal/Images.xcassets/btnAttachments--blue.imageset/Contents.json

@@ -1,23 +0,0 @@
-{
-  "images" : [
-    {
-      "idiom" : "universal",
-      "filename" : "btnAttachments--blue.png",
-      "scale" : "1x"
-    },
-    {
-      "idiom" : "universal",
-      "filename" : "btnAttachments--blue@2x.png",
-      "scale" : "2x"
-    },
-    {
-      "idiom" : "universal",
-      "filename" : "btnAttachments--blue@3x.png",
-      "scale" : "3x"
-    }
-  ],
-  "info" : {
-    "version" : 1,
-    "author" : "xcode"
-  }
-}