Removed explicitly date field in gemspec, it will assign by rubygems.org

Author: hsbt

json_pure.gemspec

@@ -7,7 +7,6 @@ Gem::Specification.new do |s|
   s.required_rubygems_version = Gem::Requirement.new(">= 0".freeze) if s.respond_to? :required_rubygems_version=
   s.require_paths = ["lib".freeze]
   s.authors = ["Florian Frank".freeze]
-  s.date = "2020-06-25"
   s.description = "This is a JSON implementation in pure Ruby.".freeze
   s.email = "[email protected]".freeze
   s.extra_rdoc_files = ["README.md".freeze]