ovn: Remove top ovn directory from PATHs.

ovn: Remove top ovn directory from PATHs.

Since 5b5c922b0ca6 ("ovn-nbctl: Move ovn-nbctl to utilities directory.")
there is no more executables in top ovn directory.

Removing of this directory from PATHs helps to avoid problems when
old executable ./ovn/ovn-nbctl used instead of ./ovn/utilities/ovn-nbctl.

This may happen if source directory was updated to commit 5b5c922b0ca6
without calling 'make clean'.

Fixes: 5b5c922b0ca6 ("ovn-nbctl: Move ovn-nbctl to utilities directory.")
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Russell Bryant <russell@ovn.org>