Bug 1815021 - Back out reading provenance data from AttributionCode r=nalexander

Backed out Bug 1815021's patch: https://hg.mozilla.org/mozilla-central/rev/9b8c31e9eb9f

Differential Revision: https://phabricator.services.mozilla.com/D189594

Author: bytesized

browser/components/attribution/AttributionCode.sys.mjs

@@ -8,7 +8,6 @@
 export const AttributionIOUtils = {
   write: async (path, bytes) => IOUtils.write(path, bytes),
   read: async path => IOUtils.read(path),
-  readUTF8: async path => IOUtils.readUTF8(path),
   exists: async path => IOUtils.exists(path),
 };