Fix #1036: Remove OSX-specific close button CSS for Heartbeat bar.
Fix #1036: Remove OSX-specific close button CSS for Heartbeat bar.

Due to changes in browser styles for Photon, the fix is no longer necessary, and
in fact makes the close button look wrong. Without the fix, the close button
looks and behaves the same as the tab bar close buttons.

mozilla/normandy,mozilla/normandy,mozilla/normandy,mozilla/normandy