Remove gfix -cache option

[mrotteveel]

Gfix currently lists a -cache option which is documented as "shutdown cache manager"), and this option - looking at the alice/exe.cpp code - can be used in combination with -shutdown, but it doesn't seem to be actually doing anything (the associated isc_dpb_shutdown bitmap value of isc_dpb_shut_cache is never used in Firebird).

This seems to be some remnant from the InterBase days, and as such, it should be removed.