Remove TickedAsyncExecutor::spawn API

[coder137]

Blocking resolution of: rust-lang/miri#3796

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.59%. Comparing base (45c76da) to head (cddc751).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
- Coverage   98.71%   98.59%   -0.13%     
==========================================
  Files           4        4              
  Lines          78       71       -7     
==========================================
- Hits           77       70       -7     
  Misses          1        1              

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