remove github.com/spf13/pflag

[ichiban]

flag is just fine.

[codecov-commenter]

Codecov Report

Merging #124 (62fabe7) into main (929e3f5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #124   +/-   ##
=======================================
  Coverage   85.63%   85.63%           
=======================================
  Files          18       18           
  Lines        5074     5074           
=======================================
  Hits         4345     4345           
  Misses        648      648           
  Partials       81       81           

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