Removed the DBMPWD (aka. DBM) define. It was per default enabled, anyway.

Removed the DBMPWD (aka. DBM) define. It was per default enabled, anyway.
The explanation of this option is:
    For sites with a dbm-style password file (such as with Yellow Pages),
    MH will not read the entire passwd file into a cache.  At one site
    that runs YP on a large passwd file, using this showed a 6:1 performance
    improvement.
I don't like performance optimization, but if the non-cached version is
faster, then we surely take that one.