FormatJson: Remove speculative comment

Follows-up bec7e8287c69. The comment "Can be removed once we require
PHP >= 5.4.28, 5.5.12, 5.6.0" relies on some assumptions that might
later prove to be incorrect:

* That the fix won't be reverted from any of those PHP versions
  (e.g. if deemed to break BC)

* That the bug will be fixed in PECL jsonc and jsond, as well as in
  HHVM

* That we don't need to support older versions of those once we
  require one of the mentioned PHP versions

Change-Id: I67034c561d54d37dee961ada8c9cf5ccfd113da1
Merge "FormatJson: Remove speculative comment"