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>
diff --git a/tools/genboardscfg.py b/tools/genboardscfg.py
index 23c956b..c2efad5 100755
--- a/tools/genboardscfg.py
+++ b/tools/genboardscfg.py
@@ -21,7 +21,6 @@
 import multiprocessing
 import optparse
 import os
-import subprocess
 import sys
 import tempfile
 import time