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.

interfaces/Fingerprintd-Store.cppm

@@ -71,13 +71,12 @@ export namespace fingerprintd::store {
     // We choose the caller's Linux uid, which makes the mapping total and
     // needs no allocation table.
     //
-    // The dev phone's existing template sits under gid 60, which was never a
-    // decision: it came from the ENROLL token's timeout field being read as
-    // the gid, and was then made self-consistent. It is kept readable as a
-    // legacy group so an enrolled finger is not stranded, but nothing new is
-    // written there.
+    // The research harness used gid 60, which was never a decision -- it was
+    // the ENROLL token's timeout field being read as the gid, then made
+    // self-consistent. There is no compatibility path for it: the finger it
+    // enrolled gets re-enrolled under the uid. Nothing is worth carrying a
+    // second group id for.
     using Gid = std::uint32_t;
-    inline constexpr Gid LegacyGid = 60;
     inline Gid GidForUid(std::uint32_t uid) { return uid; }
 
     // common.max_enrolling_fingers, from the trustlet config we ship.

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");
     }