ppc_booke: drop useless assignment

The tb_env variable is set two lines above. So just drop the double assignment.
Signed-off-by: NKONRAD Frederic <fred.konrad@greensocs.com>
Reviewed-by: NLaurent Vivier <laurent@vivier.eu>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

hw/ppc/ppc_booke.c

@@ -282,7 +282,6 @@ void store_booke_tcr(CPUPPCState *env, target_ulong val)
     ppc_tb_t *tb_env = env->tb_env;
     booke_timer_t *booke_timer = tb_env->opaque;
 
-    tb_env = env->tb_env;
     env->spr[SPR_BOOKE_TCR] = val;
     kvmppc_set_tcr(cpu);