Remove helptags test from travis

This is tested in test/help.vim

Author: thinca

scripts/run-lint-on-ci.sh

@@ -35,8 +35,5 @@ go get -d -v ./scripts
 # Run reviewdog.
 reviewdog -reporter=github-pr-check
 
-# Check tag name conflicts
-vim --cmd "try | helptags doc/ | catch | cquit | endtry" --cmd quit
-
 # Validate changelog
 ruby scripts/check-changelog.rb