AUTO_OBJ: Revert r325351 now that the fix is in auto.obj.mk in r325404.

Forcing MK_AUTO_OBJ to no is not really needed since bsd.obj.mk is protected
against 'rm -rf ${.CURDIR}' already. It was also flawed as if MK_AUTO_OBJ=yes
was in the .MAKEOVERRIDES already then it just remained on.

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