[build] Remove obsolete "allzroms" target

The .zrom format has not existed since around 2007.  ROMs are now
compressed by default.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

 
 # The "allXXXs" targets for each suffix
 #
-allall: allroms allzroms allpxes allisos alldsks
-allroms allzroms : all%s : $(foreach ROM,$(ROMS),$(BIN)/$(ROM).%)
+allall: allroms allpxes allisos alldsks
+allroms : all%s : $(foreach ROM,$(ROMS),$(BIN)/$(ROM).%)
 allpxes allisos alldsks : all%s : $(foreach DRIVER,$(DRIVERS),$(BIN)/$(DRIVER).%)
 
 # Alias for ipxe.%