ui-refs.c: Simplify and inline cmp_age()

In comparison functions, only the sign of the return value matters -- no
need to have extra checks to ensure the return value always is +/-1 when
the actual parameters differ. After removing all the checks, the
function body boils down to

    return age2 - age1;

so we can as well replace all invocations with that subtraction.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>