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Avoid unpatching bytecode twice after a trace flush.

Reported by Sergey Kaplun.

(cherry picked from commit 85c3f2fb6)

When flushing the already flushed trace, it is possible that another
trace is recorded for the same bytecode as the first trace. In that
case, the assertion fails in `trace_unpatch()`.

This patch fixes it by unpatching the trace only in the case if it
persists in the trace chain. Also, it deletes the dead code for the
trace unpatching logic, since the root trace can't start from `BC_JMP`
and the child traces are handled differently.

Sergey Kaplun:
* added the description and the test for the problem

Part of tarantool/tarantool#11055

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