[plt-scheme] Memory limit is on by default in DrScheme pre-release version 4.0.2.3

While I'm not sure whether this is a bug, I was surprised to discover
that the memory limit is on by default in the current DrScheme
pre-release version 4.0.2.3.  It was off by default in both
pre-release version 3.99.0.25, and official release version 4.0.2.

Setting the memory limit on by default may potentially cause problems
for new users who are not used to this feature, and who don't fully
understand the implications of a memory limit.

-- Benjamin L. Russell