rpc: Remove unused solana-sdk dependency

[steviez]

Problem

@t-nelson already did all of the actual work in removing use of solana-sdk usage from solana-rpc, this just removes solana-sdk from the dependency list in Cargo.toml

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.7%. Comparing base (f3a45e7) to head (47a7f36).

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #6301     +/-   ##
=========================================
- Coverage    82.8%    82.7%   -0.1%     
=========================================
  Files         845      845             
  Lines      377826   377826             
=========================================
- Hits       312851   312809     -42     
- Misses      64975    65017     +42     

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