logging: remove comment about LogLevel::ERROR
logging: remove comment about LogLevel::ERROR

Summary:
The logging code used to support a LogLevel::ERROR value on non-Windows
platforms.  I removed it in D5288600 to make all platforms consistent, but I
forgot to remove this comment describing it.

Reviewed By: bolinfest

Differential Revision: D6341244

fbshipit-source-id: 054427d342066f354c859b12611b907dc2d4fa35

facebook/folly,facebook/folly,facebook/folly,rklabs/folly,rklabs/folly,rklabs/folly,facebook/folly,rklabs/folly,rklabs/folly,facebook/folly