misc: Fix precommit install (#634)

Previously, the `subprocess` module was used to execute shell command
installing precommit hook. However, after #431 [1], the import of the
`subprocess` module was overriden by `asyncio.subprocess`, which has a
different API to execute the shell command. This change removes the
`asyncio.subprocess` import.

[1] https://github.com/gem5/gem5/pull/431

Change-Id: I9a7d51f85518089d258ab57c5d849a36dcf128e9

Signed-off-by: Hoa Nguyen <hn@hnpl.org>

site_scons/site_tools/git.py

@@ -41,7 +41,6 @@
 import os
 import subprocess
 import sys
-from asyncio import subprocess
 
 import gem5_scons.util
 import SCons.Script