cpu-exec: drop dead assignment

All uses of TB inside cpu_exec are dominated by "tb = tb_find_fast(env)",
and there are no uses after the switch statement.  So the assignment
is dead, as reported by Coverity.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

cpu-exec.c

@@ -494,7 +494,6 @@ int cpu_exec(CPUArchState *env)
                          * interrupt_request) which we will handle
                          * next time around the loop.
                          */
-                        tb = (TranslationBlock *)(next_tb & ~TB_EXIT_MASK);
                         next_tb = 0;
                         break;
                     case TB_EXIT_ICOUNT_EXPIRED: