Remove the "" entry from search_keywords.

Having this entry made it appear as if there was a search criterion
named for the empty string; i.e., a string like ":foobar" would be
parsed as an operator "" with an argument "foobar". There was no match
function defined for the empty string, which led to this crash:

Version: 6.25
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/zenmapGUI/ScanInterface.py", line 247, in filter_hosts
    self.inventory.apply_filter(filter_string)
  File "/usr/lib/python2.7/dist-packages/zenmapCore/NetworkInventory.py", line 502, in apply_filter
    if not self._match_all_args(host, operator, args):
  File "/usr/lib/python2.7/dist-packages/zenmapCore/NetworkInventory.py", line 452, in _match_all_args
    if positive != self.__getattribute__("match_%s" % operator)(host, arg):
AttributeError: 'FilteredNetworkInventory' object has no attribute 'match_'

I did some quick tests and plain keyword searching (with no colon) seems
to still work. I'm not sure why the "" entry was ever present.

Reported by Kris Paernell.
http://seclists.org/nmap-dev/2013/q3/38

zenmap/zenmapCore/NetworkInventory.py

@@ -445,7 +445,6 @@ class FilteredNetworkInventory(NetworkInventory):
         search_keywords["ofp"] = "open_filtered"
         search_keywords["closed|filtered"] = "closed_filtered"
         search_keywords["cfp"] = "closed_filtered"
-        search_keywords[""] = ""
         self.search_parser = SearchParser(self, search_keywords)
 
     # FIXME: This method doesn't do anything.  We just need to support