site_tests: remove client/site_tests/__init__.py to prevent conflicts

For site specific tests I use an external self-contained git repo in
client/site_tests/.git

This __init__.py conflicts with this external repo.  Remove it.  This
should have the side effect of preventing git from creating the
directory client/site_tests since git does not track directories, only
files.  This is fine because site_tests should not be version controlled
by autotest-core.

Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>