Don't cause --enable-cassert to define COPY_PARSE_PLAN_TREES automatically

Don't cause --enable-cassert to define COPY_PARSE_PLAN_TREES automatically
anymore.  That won't teach us anything new for the rest of this release
cycle, so it seems better to keep the --assert environment more like the
non-assert environment for beta.
I'm going to leave CLOBBER_FREED_MEMORY and MEMORY_CONTEXT_CHECKING
turned on by --enable-cassert for now, however.