remove unneeded String#mb_chars from SearchHelper

Test fails randomly.

<pre>
  1) Error:
SearchHelperTest#test_highlight_multibyte:
ArgumentError: invalid byte sequence in UTF-8
    app/helpers/search_helper.rb:36:in `block in highlight_tokens'
    app/helpers/search_helper.rb:26:in `each'
    app/helpers/search_helper.rb:26:in `each_with_index'
    app/helpers/search_helper.rb:26:in `highlight_tokens'
    test/unit/helpers/search_helper_test.rb:46:in `test_highlight_multibyte'
</pre>

String#mb_chars is for Ruby 1.8 compatibility.
Rails4 dropped Ruby 1.8 support.

git-svn-id: http://svn.redmine.org/redmine@13414 e93f8b46-1217-0410-a6f0-8f06a7374b81