fetch2/local.py: avoid using PREMIRROR
The PREMIRROR isn't useful for "file://", so avoid using it, this is
good for searching speed and can reduce useless lines in log.do_fetch.
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>