TRIV: Delete commented-out code
DOC TRIV: Delete old comment

The comment was about the assumption of there being at least one system
goal.  While this is still true in the sense that synthesize() assumes
sys_goals is non-NULL, the comment now is confusing because input files
can have no SYSGOAL or something trivial, e.g., []<>False.
The comment was originally written to distinguish from the possibility
of there being no environment goals represented by env_goals = NULL.
Improve clean-up code in several places to avoid BDD ref leaks

E.g., before this commit, leaks can be observed by

  ./gr1c -vv -t txt examples/counter.spc

which shows the reference count to be greater than zero.