#4926 asksign uses facts even after they've been deleted

[Stavros Macrakis]

asksign results aren't deleted by forget although facts says they are

[ sign(asin(x)),
  asksign(asin(x)),
  sign(asin(x)),
  forget(facts()),
  facts(),
  sign(asin(x)),
  asksign(asin(x)) ]
=>
[pnz,   << OK
 pos,    << asks, OK
 pos,    << uses asksign addition to database
 [[x > 0]],
 [],     << fact is seemingly deleted
 pnz,    << sign acts as though fact is deleted
 pos ]    << but asksign does not ask and gives cached result

Another display of these results:

map('display,
   '[ sign(asin(x)), asksign(asin(x)), sign(asin(x)), forget(facts()), facts(), sign(asin(x)), asksign(asin(x)) ]);

sign(asin(x)) = pnz
     Is x positive, negative or zero?   << goes with following asksign
     p;
asksign(asin(x)) = pos
sign(asin(x)) = pos
forget(facts()) = [[x > 0]]
facts() = []
sign(asin(x)) = pnz
asksign(asin(x)) = pos

Tested in Maxima 5.49.0 Lisp SBCL 2.6.0