[F] remove unused var c0. Avoid linter warnings.

[dvarrui]

Running my project test appears this warning:

/home/quigon/.rbenv/versions/3.1.0/lib64/ruby/gems/3.1.0/gems/unicode_utils-1.4.0/lib/unicode_utils/each_grapheme.rb:37: warning: assigned but unused variable - c0
Loaded suite /home/quigon/.rbenv/versions/3.1.0/lib64/ruby/gems/3.1.0/gems/rake-13.0.6/lib/rake/rake_test_loader
Started

My suggestion is delete lines with c0 var, This var isn't used.

Thanks!