samples: drivers: can: increase the maximum number of shell arguments

Remove the custom Kconfig value for CONFIG_SHELL_ARGC_MAX as it is too low
to account for the new arguments to the "can send" shell command.

The default for CONFIG_SHELL_ARGC_MAX was increased to 20 in
32ebeb0a which is sufficient for sending
non-CAN-FD format frames using the new "can send" shell command.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>