Debugger: Re-enable thread view sorting

This was disabled as workaround for QTCREATORBUG-7432 but can
be supported again after fefbc172f.

Task-number: QTCREATORBUG-7432
Change-Id: Ic85f6f91c0f6f04dc76f66984cb5773b0753ad26
Reviewed-by: Orgad Shaneh <orgads@gmail.com>