Removed nonsense `rubygems_version` in input gemspec files

As it is ignored and set at building packages automatically, it is
just nonsense to set in gemspec file for input.

ext/json/json.gemspec

@@ -131,7 +131,6 @@ Gem::Specification.new do |s|
   s.licenses = ["Ruby"]
   s.rdoc_options = ["--title", "JSON implemention for Ruby", "--main", "README.md"]
   s.required_ruby_version = Gem::Requirement.new(">= 2.0")
-  s.rubygems_version = "3.0.3"
   s.summary = "JSON Implementation for Ruby"
   s.test_files = ["tests/test_helper.rb"]
 

ext/psych/psych.gemspec

@@ -46,7 +46,6 @@ DESCRIPTION
   s.extra_rdoc_files = ["README.md"]
 
   s.required_ruby_version = Gem::Requirement.new(">= 2.4.0")
-  s.rubygems_version = "2.5.1"
   s.required_rubygems_version = Gem::Requirement.new(">= 0")
 
   if RUBY_ENGINE == 'jruby'

ext/stringio/stringio.gemspec

@@ -24,7 +24,6 @@ Gem::Specification.new do |s|
   s.homepage = "https://github.com/ruby/stringio"
   s.licenses = ["BSD-2-Clause"]
   s.required_ruby_version = ">= 2.5"
-  s.rubygems_version = "2.6.11"
   s.summary = "Pseudo IO on String"
 
   # s.cert_chain  = %w[certs/nobu.pem]

lib/racc/racc.gemspec

@@ -82,7 +82,6 @@ DESC
     "test/testscanner.rb", "web/racc.en.rhtml", "web/racc.ja.rhtml"
   ]
   s.require_paths = ["lib"]
-  s.rubygems_version = "3.1.0.pre1"
   s.required_rubygems_version = Gem::Requirement.new(">= 0") if s.respond_to? :required_rubygems_version=
   s.rdoc_options = ["--main", "README.rdoc"]
   s.extra_rdoc_files = [

lib/rdoc/rdoc.gemspec

@@ -241,6 +241,5 @@ RDoc includes the +rdoc+ and +ri+ tools for generating and displaying documentat
   ]
 
   s.required_ruby_version = Gem::Requirement.new(">= 2.4.0")
-  s.rubygems_version = "2.5.2"
   s.required_rubygems_version = Gem::Requirement.new(">= 2.2")
 end