Makes env keys and named accounts explicit

[penandlim]

Replace env variable ACCOUNT_PRIVATE_KEYS with DEPLOYER_PRIVATE_KEY Allows impersonating the saddle deployer address by default when the private key is not given. Remove unnecessary deploy script

[codecov[bot]]

Codecov Report

Merging #776 (1f61a46) into master (64d29f5) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff            @@
##           master     #776    +/-   ##
========================================
  Coverage   92.95%   92.95%            
========================================
  Files          52       52            
  Lines        3050     3050            
  Branches      508      632   +124     
========================================
  Hits         2835     2835            
  Misses        215      215            

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