expr: remove support for evaluating string as integer

expr: remove support for evaluating string as integer

As the comment nicely puts it, this is a bit weird. When you try to
evaluate an expression of type string into an integer, what it does is:
    "" -> 0
    "c" -> (ascii value, i.e. like a char literal)
    more than one char -> error
The first one is obviously not very useful; why not just write 0?
The second one might be useful (though I don't see where in a keymap
it would be), but I don't think anyone would consider trying "X" for
that anyway.

A look through xkeyboard-config shows "" only used once as a string, and
"X" also only used as strings (and mostly in geometry which we don't
evaluate anyway). And I seriously doubt it's used (purposely) anywhere
else. So remove it.

Signed-off-by: Ran Benita <ran234@gmail.com>