Turn metp back off for extended test

The metplus job isn't running correctly (it "succeeds" but has a
ton of silent errors), so turning it off until it works. May be as
simple as the archive not working properly for gfs output, but there
may be more underneath, as these haven't been validated in a while.

Refs: NOAA-EMC#2673

ci/cases/yamls/gfs_extended_ci.yaml

@@ -9,6 +9,5 @@ base:
   DO_AWIPS: "NO"
   DO_NPOESS: "YES"
   DO_GENESIS_FSU: "NO"
-  DO_METP: "YES"
   FHMAX_GFS: 384
   FHMAX_HF_GFS: 120