mpc83xx: Remove stale CONFIG_SYS_LBLAWBAR{4/5/6/7}_PRELIM

Last (incorrect) use of those CONFIG items was removed by
commit 9fd9abed ("TQM834x: remove defines causing gcc4.4 warnings")

Those items are invalid and should have been removed at the
same time because lblaw[] has only 4 elements.

And they were removed from the whitelist by
commit 9c5df7a2 ("mpc83xx: Migrate LBLAW_* to Kconfig")

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Fixes: 9fd9abed ("TQM834x: remove defines causing gcc4.4 warnings")