Revert "build(deps): add base64 as a development dependency"

This reverts commit 8c8522b.

`base64` is not needed since we updated the `rubocop` version, and the
latest versions of `rubocop` has `base64` as a dependency.

cose.gemspec

@@ -35,7 +35,6 @@ Gem::Specification.new do |spec|
   spec.add_dependency "openssl-signature_algorithm", "~> 1.0"
 
   spec.add_development_dependency "appraisal", "~> 2.2.0"
-  spec.add_development_dependency "base64", "~> 0.2"
   spec.add_development_dependency "bundler", ">= 1.17", "< 3"
   spec.add_development_dependency "byebug", "~> 11.0"
   spec.add_development_dependency "rake", "~> 13.0"