stress2: exlock2.sh: Preserve behavior after ps(1)'s '-U' change

Remove '-Uroot', as now '-U' selects processes based on their real user
IDs, and since the program using them was launched as 'root', the
equivalent now is just to remove it, as now ps(1) by default selects
processes based on its effective UID (root) and their effective UIDs.

It seems that matching on effective or real UID does not really matter
in this test, but at least this change simplifies the command-line.

MFC after: 1 day
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50200