Remove is_khtml again

Remove is_khtml again

This variable is completely misnamed, since it matches WebKit as well.
It's also almost certainly completely useless, since the probability of
some bug existing in all versions of KHTML/WebKit past, present, and
future is approximately zero.  Use capability testing if at all
possible; and if it's not possible, then at least check "browser X
version <= y".  *Never* check for just "browser X"!  That will always
break if a new, fixed version comes out.