chore: remove placeholder file from longrunning

[zhumin8]

This file looks like a leftover from #198. Other placeholder files created in the same PR were removed, e.g. src/generated/type/src/main.rs, src/generated/cloud/secretmanager/v1/src/main.rs.

Encountered this file when comparing generate results, see #4121.

Let me know if I am misunderstanding or missing context.

[dbolduc]

Thanks!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.78%. Comparing base (cde7484) to head (92aff11).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4122   +/-   ##
=======================================
  Coverage   94.78%   94.78%           
=======================================
  Files         181      181           
  Lines        6866     6866           
=======================================
  Hits         6508     6508           
  Misses        358      358           

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