Fix undef internal call in supervisor2

[gomoripeti]

Proposed Changes

Must be a leftover from the refactoring in commit 0a87aea

This should prevent the below crash that was seen with an exchange federation link

{undef,
    [{supervisor2,try_again_restart,
         [<0.105395.0>,
          {upstream,
              [{encrypted,
                   <<"...">>}],
              <some mirrored supervisor data>},
          {upstream,
              [{encrypted,
                   <<"...">>}],
              <some mirrored supervisor data>}],
         []}]}

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Further Comments

[gomoripeti]

timer:apply_after with zero timeout executes the function immediatly but in a spawned process. However try_again_restart/1 only sends a gen_server cast so it should be fine to call it from the same process (same way as from restart/2)

[gomoripeti]

(test failures look unrelated)