Don't call utime() on the node.dab file for every read

... this was the cause of some observed unnecessarily high disk/file server
(Samba) utilization, as we call getnodedat() a lot. utime() opens and closes
the file, which was already open - and we're not modifying the file, so
updating the 'modification time' here is wrong anyway.

Disabling this 21-year old bit of logic resulted in a pretty dramatic
reduction in Samba (smbd) CPU utilization on Vertrauen.

If a BBS actually needes this hack (e.g. for NFS compatibility, as Deuce
eluded in the comment), they'd be better off just setting the "Keep Node File
Open" node setting (in SCFG->Nodes) to "No".