Simplify the tmt plan a bit

Remove the unnecessary `/test` key hierarchy.
The plan name does not have to be `ci.fmf`.
Execute method `beakerlib` has been deprecated.

See the tmt guide for more details:
https://tmt.readthedocs.io/en/latest/guide.html#plans

ci.fmf

@@ -1,13 +1,11 @@
-/test:
-    summary:
+summary:
     CI plan, picks Tier1 tests, runs in beakerlib.
-    description:
-        This special plan has to be named ci.fmf to run in CI.
-        It selects test which have 'tier 1' in their main.fmf file
-    discover:
+description:
+    It selects tests which have 'tier 1' in their main.fmf file.
+discover:
   - name: tier1
     how: fmf
     filter: 'tier: 1'
-        url: "https://src.fedoraproject.org/tests/ftp.git"
-    execute:
-        how: beakerlib
+    url: https://src.fedoraproject.org/tests/ftp.git
+execute:
+    how: tmt