Deprecate prep kwarg and remove third list from process_queue

[lillian542]

The empty third list returned by process_queue is removed, and a deprecation warning is added when the prep kwarg is used in QuantumScript

[lillian542]

[sc-43884]

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (90f09d4) 99.66% compared to head (de4aa54) 99.66%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4554   +/-   ##
=======================================
  Coverage   99.66%   99.66%           
=======================================
  Files         376      376           
  Lines       33070    33071    +1     
=======================================
+ Hits        32960    32961    +1     
  Misses        110      110           

Files Changed	Coverage Δ
pennylane/queuing.py	100.00% <100.00%> (ø)
pennylane/tape/operation_recorder.py	96.77% <100.00%> (ø)
pennylane/tape/qscript.py	100.00% <100.00%> (ø)
pennylane/tape/tape.py	98.55% <100.00%> (ø)

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[timmysilv]

please uncomment this pytest.warns and indent the line below it to catch this warning. One extra comment in-line, otherwise looks good!