Remove unimplemented user override of DefaultSummaryRows

479a311 added DefaultSummaryRows to the lsit of user-overridable
options, but failed to change any of the calls to
->Get('DefaultSummaryRows') to respect the user's setting.  Remove the
value from the list of user-configurable settings, as it would merely
serve as a default for the per-user preference already found on the
/Prefs/MyRT.html page.  Having a per-user configurable default setting
for a per-user configurable preference makes no sense.
Merge branch '4.0/delete-default-summary-rows-pref' into 4.0-trunk