update: stop extracting and storing XML icons, they're useless

APKs that only have XML icons in them should include an icon.png in the
metadata or fastlane/triple-t

closes #344

tests/repo/index-v1.json

@@ -54,7 +54,6 @@
       "name": "Caffeine Tile",
       "summary": "Test app for extracting icons when an XML one is default",
       "added": 1539129600000,
-      "icon": "info.zwanenburg.caffeinetile.4.xml",
       "packageName": "info.zwanenburg.caffeinetile",
       "lastUpdated": 1539129600000
     },

tests/repo/index.xml

@@ -53,7 +53,6 @@ Features:
 		<lastupdated>2018-10-10</lastupdated>
 		<name>Caffeine Tile</name>
 		<summary>Test app for extracting icons when an XML one is default</summary>
-		<icon>info.zwanenburg.caffeinetile.4.xml</icon>
 		<desc>No description available</desc>
 		<license>Unknown</license>
 		<categories>Development</categories>

tests/update.TestCase

@@ -707,8 +707,7 @@ class UpdateTest(unittest.TestCase):
 
         apk_info = fdroidserver.update.scan_apk('repo/info.zwanenburg.caffeinetile_4.apk')
         self.assertEqual(apk_info.get('versionName'), '1.3')
-        self.assertEqual(apk_info['icons_src'], {'160': 'res/drawable/ic_coffee_on.xml',
-                                                 '-1': 'res/drawable/ic_coffee_on.xml'})
+        self.assertEqual(apk_info['icons_src'], {})
 
         apk_info = fdroidserver.update.scan_apk('repo/com.politedroid_6.apk')
         self.assertEqual(apk_info.get('versionName'), '1.5')