Check for folder in sync_default_gems (ruby#15933)

It was not clear to me that you have to do anything for this command to work.
Previous versions (for example on the 3.4 branch) had this check
but it got lost along the way.

Without this when the folder doesn't exist, you get this error (after it deleted all the files):
```
$ ./tool/sync_default_gems.rb syntax_suggest
Sync ruby/syntax_suggest
./tool/sync_default_gems.rb:464:in 'SyncDefaultGems.check_prerelease_version': undefined method 'version' for nil (NoMethodError)

    puts "#{gem}-#{spec.version} is not latest version of rubygems.org" if spec.version.to_s != latest_version
                                                                               ^^^^^^^^
        from ./tool/sync_default_gems.rb:436:in 'SyncDefaultGems.sync_default_gems'
        from ./tool/sync_default_gems.rb:942:in '<module:SyncDefaultGems>'
        from ./tool/sync_default_gems.rb:10:in '<main>'
```

Now you get

```
$ ./tool/sync_default_gems.rb syntax_suggest
Sync ruby/syntax_suggest
Expected '../ruby/syntax_suggest' (/home/earlopain/Documents/ruby/syntax_suggest)  to be a directory, but it didn't exist.
```

This was changed in ruby@b722631

Since then, `sync_lib` is unused, delete it

Author: Earlopain

tool/sync_default_gems.rb

@@ -398,6 +398,10 @@ def sync_default_gems(gem)
 
     upstream = File.join("..", "..", config.upstream)
 
+    unless File.exist?(upstream)
+      abort %[Expected '#{upstream}' (#{File.expand_path("#{upstream}")}) to be a directory, but it didn't exist.]
+    end
+
     config.mappings.each do |src, dst|
       rm_rf(dst)
     end
@@ -798,26 +802,6 @@ def sync_default_gems_with_commits(gem, ranges, edit: nil)
     return true
   end
 
-  def sync_lib(repo, upstream = nil)
-    unless upstream and File.directory?(upstream) or File.directory?(upstream = "../#{repo}")
-      abort %[Expected '#{upstream}' \(#{File.expand_path("#{upstream}")}\) to be a directory, but it wasn't.]
-    end
-    rm_rf(["lib/#{repo}.rb", "lib/#{repo}/*", "test/test_#{repo}.rb"])
-    cp_r(Dir.glob("#{upstream}/lib/*"), "lib")
-    tests = if File.directory?("test/#{repo}")
-              "test/#{repo}"
-            else
-              "test/test_#{repo}.rb"
-            end
-    cp_r("#{upstream}/#{tests}", "test") if File.exist?("#{upstream}/#{tests}")
-    gemspec = if File.directory?("lib/#{repo}")
-                "lib/#{repo}/#{repo}.gemspec"
-              else
-                "lib/#{repo}.gemspec"
-              end
-    cp_r("#{upstream}/#{repo}.gemspec", "#{gemspec}")
-  end
-
   def update_default_gems(gem, release: false)
     config = REPOSITORIES[gem]
     author, repository = config.upstream.split('/')