sql: refactor sqlVdbeMemIntegerify() function

The sqlVdbeMemIntegerify() function accepts the is_forced flag as
one of the arguments. But this flag is actually useless, because
regardless of whether it is TRUE or FALSE, the function will do
the same. This patch removes the is_forced argument from the
function. There will be no test, since the result of the function
has not changed.

Part of #4526