Remove xcpretty-travis-formatter from Rakefile

The Travis CI formatter is unused — CI runs on Buildkite.
The Rakefile now uses plain `xcpretty` instead.

---

Generated with the help of Claude Code, https://claude.ai/code

Co-Authored-By: Claude Code Opus 4.6 <noreply@anthropic.com>

Author: mokagio

Rakefile

@@ -192,7 +192,7 @@ def xcodebuild(*build_cmds)
   cmd += " -configuration #{xcode_configuration}"
   cmd += ' '
   cmd += build_cmds.map(&:to_s).join(' ')
-  cmd += ' | bundle exec xcpretty -f `bundle exec xcpretty-travis-formatter` && exit ${PIPESTATUS[0]}' unless ENV['verbose']
+  cmd += ' | bundle exec xcpretty && exit ${PIPESTATUS[0]}' unless ENV['verbose']
   sh(cmd)
 end