Remove unnecessary g++ configuration (#6499)

.travis.yml

@@ -23,7 +23,6 @@ env:
     - RAILS_ENV=test
     - NOKOGIRI_USE_SYSTEM_LIBRARIES=true
     - PARALLEL_TEST_PROCESSORS=2
-    - "PATH=$HOME:$PATH"
 
 addons:
   postgresql: 9.4
@@ -53,7 +52,6 @@ install:
 
 before_script:
   - ./bin/rails parallel:create parallel:load_schema parallel:prepare assets:precompile
-  - ln -s /usr/bin/x86_64-linux-gnu-g++-6 "$HOME/g++"
 
 script:
   - travis_retry bundle exec parallel_test spec/ --group-by filesize --type rspec