bin: remove unsupported %b stats from --help

* bin/common_aoutput.cc: Here.
* NEWS: Mention it.

NEWS

@@ -66,6 +66,9 @@ New in spot 2.3.3.dev (not yet released)
   - the transformation to state-based acceptance (spot::sbacc()) was
     incorrect on automata where the empty acceptance mark is accepting.
 
+  - the --help output of randaut and ltl2tgba was showing an
+    unsupported %b stat.
+
   Backward-incompatible changes:
 
   - spot::acc_cond::mark_t::operator bool() has been marked as

bin/common_aoutput.cc

@@ -239,8 +239,6 @@ static const argp_option o_options[] =
       "number of transitions", 0 },
     { "%a", 0, nullptr, OPTION_DOC | OPTION_NO_USAGE,
       "number of acceptance sets", 0 },
-    { "%b", 0, nullptr, OPTION_DOC | OPTION_NO_USAGE,
-      "number of acceptance sets", 0 },
     { "%g", 0, nullptr, OPTION_DOC | OPTION_NO_USAGE,
       "acceptance condition (in HOA syntax)", 0 },
     { "%c, %[LETTERS]c", 0, nullptr, OPTION_DOC | OPTION_NO_USAGE,