useradd.bblass: Simplify target overrides

useradd.bblass: Simplify target overrides

The current style might be a leftover from when _class-target did not
exist.

Also change the assignment to SSTATECLEANFUNCS to an append, which makes
more sense. useradd.bbclass is the only user of SSTATECLEANFUNCS as of
writing, so it won't make any functional difference.

(From OE-Core rev: 79dd6be736211a722538a1234337ca16fefd5540)

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>