decodetree: Drop check for less than 2 patterns in a group

While it makes little sense for the end product to have a group
containing only a single pattern, avoiding this case within an
incremental patch set is troublesome.

Because this is expected to be a transient condition, do not
bother "optimizing" this case, e.g. by folding away the group.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

tests/decode/succ_pattern_group_nest4.decode

@@ -0,0 +1,13 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Verify deeper nesting, and a single element in the groups.
+{
+  [
+    {
+      [
+        sub1  00000000 a:8 b:8 c:8
+      ]
+    }
+  ]
+}