Fix command line interface for math_brute_force (#1145)

Ensure the following forms of command lines are supported, as per usage
message (-h):
 - math_brute_force [<name1> [<name2> ... [<nameN>]]]
 - math_brute_force I [J]

Remove dead/unnecessary code.

Fix regression introduced in f337e0b6 ( Fix command-line function range
for bruteforce  (#1127), 2021-01-29).

Signed-off-by: Marco Antognini <marco.antognini@arm.com>