pr-branch-auto-delete should be disabled by default

[cmccandless]

I discovered the addition of pr-branch-auto-delete (#2186) when I noticed that branches were being deleted without my explicit action. The feature functions as designed, but as this affects user repositories directly (and not just how they are presented), this is a feature that should not be enabled by default.

[fregante]

Under what circumstances would you prefer to keep the branch?

The feature is enabled by default because: #1024 (comment)

[sindresorhus]

Also note that the action is easily reversible.