tools/genboardscfg.py: remove bogus import subprocess

Since f6c8f38ec601 ("tools/genboardscfg.py: improve performance more
with Kconfiglib"), this tool does not use the subprocess module.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

tools/genboardscfg.py

@@ -21,7 +21,6 @@ import glob
 import multiprocessing
 import optparse
 import os
-import subprocess
 import sys
 import tempfile
 import time