verification/rvgen: Do not generate unused variables

ltl2k generates all variable definition in both ltl_start() and
ltl_possible_next_states(). However, these two functions may not use all
the variables, causing "unused variable" compiler warning.

Change the script to only generate used variables.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/636b2b2d99a9bd46a9f77a078d44ebd7ffc7508c.1752850449.git.namcao@linutronix.de


Signed-off-by: Nam Cao <namcao@linutronix.de>
Reviewed-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>