git-submodule.sh: get rid of isnumber

git-submodule.sh: get rid of isnumber

It's entirely unnecessary to check whether the argument given to an
option (i.e. --summary-limit) is valid in the shell wrapper, since it's
already done when parsing the various options in git-submodule--helper.

Remove this check from the script; this both improves consistency
throughout the script, and the error message shown to the user in case
some invalid non-numeric argument was passed to "--summary-limit" is
more informative as well.

Signed-off-by: Roy Eldar <royeldar0@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>