chore(sdk): remove unused fields from a filesync message

[speezepearson]

Description

StepUpload.RequestUpload is only instantiated by FilePusher.file_changed; and half of the arguments to file_changed are never used. (A couple of these args used to be used, but were removed in 949013a in a huge mid-2020 Artifacts-overhaul.)

Testing

If I missed a place these args/fields are used, Mypy will catch it.

[codecov]

Codecov Report

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4662   +/-   ##
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  Coverage        ?   83.23%           
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  Files           ?      279           
  Lines           ?    34910           
  Branches        ?        0           
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  Hits            ?    29056           
  Misses          ?     5854           
  Partials        ?        0           

Flag	Coverage Δ
functest	56.07% <ø> (?)
unittest	73.78% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
wandb/filesync/step_checksum.py	92.42% <ø> (ø)
wandb/sdk/internal/file_pusher.py	74.64% <ø> (ø)