Now that we have added PrefRegistry registration functions that do not need to
accept a flags argument, passing PrefRegistrySyncable::UNSYNCABLE_PREF is
redundant and we can remove it altogether.

Ran the following command:
for x in $(git ls-files | grep -E "\.(cc|h|mm)$") ; do perl -0777 -i -pe 's/,[ \n]*(user_prefs::)+PrefRegistrySyncable::UNSYNCABLE_PREF//igs' $x ; done && git cl format

There are still some occurences that need to be manually fixed.

BUG=476800

Review URL: https://codereview.chromium.org/1102733002

Cr-Commit-Position: refs/heads/master@{#326993}