helper/command: drop unused variables

In both functions script_debug() and script_command_args_alloc()
the variable len is never used, and Jim_GetString() does not
mandate it.

Drop the variable and pass NULL to Jim_GetString().

Change-Id: I754b27a59c6087cde729496be42609d2a7145b0c
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8053
Tested-by: jenkins