[rubygems/rubygems] Revert "Workaround for TruffleRuby that set nil to LIBRUBY_RELATIVE"

This reverts commit rubygems/rubygems@06fd39997348, for a
bug that is fixed by rubygems/rubygems@0148cce153a5.

rubygems/rubygems@907e51e521

Author: nobu

test/rubygems/test_gem_ext_builder.rb

@@ -323,10 +323,10 @@ def self.expand(val, config = CONFIG); val; end
       end
       RUBY
       RbConfig::CONFIG.each do |k, v|
-        f.puts %(RbConfig::CONFIG[#{k.dump}] = #{v&.dump || "nil"})
+        f.puts %(RbConfig::CONFIG[#{k.dump}] = #{v.dump})
       end
       RbConfig::MAKEFILE_CONFIG.each do |k, v|
-        f.puts %(RbConfig::MAKEFILE_CONFIG[#{k.dump}] = #{v&.dump || "nil"})
+        f.puts %(RbConfig::MAKEFILE_CONFIG[#{k.dump}] = #{v.dump})
       end
       f.puts "RbConfig::CONFIG['host_os'] = 'fake_os'"
       f.puts "RbConfig::CONFIG['arch'] = 'fake_arch'"