Before this commit, when the gNB crashes, the eNB keeps running.
Same, if the eNB crashes, the gNB keeps running.

In a far past when this happened, the other program (the one not
crashing) was forced to exit.

Then this behavior was changed, for some reason.
But the code was not finished, so now we have a system in an inconsistent
state.

So either we accept that the connection between eNB and gNB can break,
that is one of the programs crashes, and we clean the state of the
program that keeps running. But this is a complex work and it will
surely not survive very long, because someone will change something
in the code later that will break this complex behavior.

Or, simpler, we go back to initial behavior, which is: the program that
did not crash does actually exit when the other crashes.

This commit provides the second solution.

It can easily be reverted whenever someone wants to implement the complex
solution.