Drop outdated pandoc-citeproc from SystemRequirements (#474)

[ishaan-arora-1]

Fixes #474.

pandoc-citeproc was replaced by built-in --citeproc in Pandoc 2.11 (Oct 2020) and no longer ships with Pandoc. The repo was archived in April 2021. CI already uses Pandoc 3.x without it.

From the Pandoc 2.11 release notes:

"It is no longer necessary to use the external pandoc-citeproc filter."

https://pandoc.org/releases.html#pandoc-2.11-2020-10-11

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.70%. Comparing base (05800bd) to head (861dc2b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #475      +/-   ##
==========================================
+ Coverage   98.66%   98.70%   +0.03%     
==========================================
  Files          35       35              
  Lines        5860     5860              
==========================================
+ Hits         5782     5784       +2     
+ Misses         78       76       -2     

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[jgabry]

Thanks!