Revert "Changed all gaia-subheaders in costcontrol to aria-level 2 with role="heading""

This reverts commit a32ad3e1cb738dbe4d4643cbbe5c9f3e5ee9d808.

Revert for not having the proper bug number in the commit message, bug 1069599.
Bug 1069599 - Changed all gaia-subheaders in costcontrol to aria-level 2 with role="heading" r=salva