Stop trying to remove every file on extraction

When we're extracting a gem, it should be extracted to an empty
directory.  Trying to remove every file before extracting the file
greatly slows the tar extraction process.

This change increases tar extraction speed by a bit over 10%:

Master branch:

```
aaron@tc ~/g/tartest (main)> ruby -I../rubygems/lib:../vernier/lib test.rb
{MEAN: 0.6701955500058829}
{stddev: 0.029875687861609046}
{"mib/s": 20.88942548167786}
```

This commit:

```
aaron@tc ~/g/tartest (main)> ruby -I../rubygems/lib:../vernier/lib test.rb
{MEAN: 0.6002237499691546}
{stddev: 0.03722422666707127}
{"mib/s": 23.324635189326408}
```

Author: tenderlove

lib/rubygems/package.rb

@@ -436,8 +436,6 @@ def extract_tar_gz(io, destination_dir, pattern = "*") # :nodoc:
           symlinks << [full_name, link_target, destination, real_destination]
         end
 
-        FileUtils.rm_rf destination
-
         mkdir =
           if entry.directory?
             destination