Check for folder in `sync_default_gems` (#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 https://github.com/ruby/ruby/commit/b722631b481314023b9fa2f3fd16fa9ab0b4bf9c

Since then, `sync_lib` is unused, delete it

0 files changed, 0 insertions, 0 deletions