Revert r58345 and r58371.

	  These changes break the behavior of default gems. Bug #13428 says
	  r58345 is reasonable because gemspec file is installed by `to_ruby_for_cache`
	  method. But I revert `to_ruby_for_cache` in rbinstall.rb at r58403.
	  There is no reason that we apply r58345 now.

	  But I'm not sure about gemspec of default gems affects standalone gems.
	  I'm going to investigate it on rubygems/rubygems.

	  [Bug #15500][ruby-core:90867]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@66913 b2dd03c8-39d4-4d8f-98ff-823fe69b080e