Clean up after "Kill mbstring fallbacks"

Clean up after "Kill mbstring fallbacks"

* Removed fallback code from Language, the associated data file
  (Utf8Case.ser), and the code to generate that data file.
* Removed comment in LanguageFi that "mb_substr has a compatibility
  function in GlobalFunctions.php".
* Removed check for mbstring in bench_utf8_title_check.php.
* In the tests for StringUtils::isUtf8():
  * Removed separate test for the non-mbstring code path.
  * Removed mentions of mbstring from function names and assertion
    messages, since mb_check_encoding() is now always used.
* Also updated the comment in StringUtils::isUtf8() referring to
  PHP 5.3, which is no longer supported in MediaWiki, to indicate
  that the same issue also exists in old versions of HHVM. (If
  we don't have to support 3.4 or older, then the function could
  be deprecated and removed if desired.)

Follows-up 943563062f0a6995.

Change-Id: I55e5cd534b849c6ea06a7fadacbbf34a12d87ebe