core: get rid of fiber_set_cancellable in hot_standby_f

Currently it's possible to wakeup a `hot_standby_f` fiber from Lua,
this does not lead to any error, but it results in redundant
`recover_remaining_wals` calls.
This patch handles such spurious wakeups in `hot_standby_f`.

Part of #7166

NO_DOC=refactoring
NO_TEST=refactoring
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