approval-voting-parallel subsystem production enablement

[alexggh]

Post #4849 merging the following tasks need to be done to consider the work completely finished:

• Stage1: enable the approval-voting-parallel by default on kusama ,enabled with Enable approval-voting-parallel by default on kusama #6218
• Stage2: enable the approval-voting-parallel by default on polkadot.
• Stage3: Follow-up cleanups, remove all the logic tied to the approval-voting-parallel flag.
• Stage4: Move all the files in a single subsystem.