Delete REQUIRE file

[DilumAluthge]

Now that there is a Project.toml file, we can delete the REQUIRE file.

[codecov-io]

Codecov Report

Merging #237 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #237   +/-   ##
=======================================
  Coverage   90.11%   90.11%           
=======================================
  Files           6        6           
  Lines         344      344           
=======================================
  Hits          310      310           
  Misses         34       34

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