Drop the gid 60 compatibility path

gid 60 was never a decision -- it was the ENROLL token's timeout field being
read as a gid, then made self-consistent. Keeping it readable alongside gid =
uid would be a second code path, a second group to reason about, and a
migration to eventually retire, all to avoid re-enrolling one finger on one dev
phone. Re-enrol it instead.

tests/Store/main.cpp

@@ -150,8 +150,7 @@ int main() {
     {
         Check(GidForUid(1000) == 1000, "gid is the uid");
         Check(GidForUid(0) == 0, "root maps too");
-        Check(LegacyGid == 60, "the dev phone's accidental group is remembered");
-        Check(GidForUid(1000) != LegacyGid, "and a real uid does not collide with it");
+        Check(GidForUid(4294967295u) == 4294967295u, "the whole uid range maps");
         Check(PathForUid("/var/lib/fingerprintd", 1000)
               == "/var/lib/fingerprintd/fingers-1000.map", "state path");
     }