[processor/batch] Removed casts to Sizer interface in batch processor

[cpheps]

Description: As a follow up to #5929 removed cast to Sizer interface in batch processor as this is no longer required with the new MarshalerSizer that is returned from NewProtoMarshaler

[TylerHelmuth]

LGTM. probably don't need a changelog entry since there isn't any user impact from that change.

[codecov]

Codecov Report

Merging #5937 (f59d884) into main (ec04503) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #5937   +/-   ##
=======================================
  Coverage   91.93%   91.93%           
=======================================
  Files         200      200           
  Lines       12406    12406           
=======================================
  Hits        11406    11406           
  Misses        789      789           
  Partials      211      211           

Impacted Files	Coverage Δ
processor/batchprocessor/batch_processor.go	90.67% <100.00%> (ø)

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[cpheps]

probably don't need a changelog entry since there isn't any user impact from that change.

@TylerHelmuth I can remove it. I think someone has to tag this PR so CI doesn't fail on not having one.