[plt-scheme] 3m defaults to --enable-account

Memory accounting (for `custodian-limit-memory', etc.) is now enabled
by default in the 3m variants of MzScheme and MrEd. In other words,
configuring with --enable-account is redundant.

If you need the old 3m collector for some reason, configure with
--enable-compact. As the name of this flag suggests, the old 3m
collector tends to keep a smaller memory footprint than the now-default
collector, but the difference between the two collectors is much less
significant than the difference between 3m "precise" collection and
conservative collection. The old collector does not support memory
accounting.

Matthew