8460b67ef8
The ability to adjust the "Backwire Opacity" was mistakenly removed in
version 2.93 (b365cc017a).
As this issue went unnoticed by most users, it appears reasonable to
completely remove this setting from the code.
By making this change, there is no longer a need to define a default
value for `View3DOverlay::backwire_opacity`.
Pull Request: https://projects.blender.org/blender/blender/pulls/116799