Remove unused libstdc++ memory env vars from couchbase-server.sh.in

We currently set a couple of env vars to force the use of new/delete
by libstdc++ in the standard startup script. Alk has confirmed that
the new_allocator is used by default (even by RHEL5). Remove these env
vars.

Change-Id: I699db40de191be43d5e4fb08aede3819b1dcff2e
Reviewed-on: http://review.couchbase.org/40113
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>