GH-47404: [Ruby] Remove needless require "extpp/setup" (#47405)

### Rationale for this change

It's no longer needed with recent extpp.

### What changes are included in this PR?

Remove needless `require "extpp/setup"`.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #47404

Authored-by: Sutou Kouhei <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: kou

ruby/red-arrow/lib/arrow/ruby.rb

@@ -15,7 +15,6 @@
 # specific language governing permissions and limitations
 # under the License.
 
-require "extpp/setup"
 require "gio2"
 
 require_relative "loader"