Remove all EOL maintenance branches

[pelson]

For the sake of cleanliness, all of our old end-of-life maintenance version branches should be removed once confirmed that they are merged back into master (i.e. so that we aren't losing commits).

In addition:

• Remove the "decoupler" branch
• Remove all merged feature branches

[djkirkham]

Currently working on removing the decoupler branch

[djkirkham]

List of branches to look at:

decoupler (dc73a1e) - has been deleted.
revert-2326-ghapim (eb867d6) - is already merged and has been deleted.
dask(9c9cd2f) - was merged in #2597 and has been deleted.
dask_prerelease_docs (8ebe939) - was merged in #2627 and has now been deleted.

[djkirkham]

Release branches:

v1.12.x (was at 316a3f4) - PR #2855
v1.11.x (76258cd) - Nothing to add. Deleted.
v1.10.x (a0940c5) - Nothing to add. Deleted.
v1.9.x (was at 5c11059) - Apparently nothing to add. PR #2854.
v1.8.x (4ba0616) - Nothing to add. Deleted.
v1.7.x (fa2f99c) - Nothing to add. Deleted.
v1.6.x (5079bc9) - Nothing to add. Deleted.
v1.5.x (a5e7ea6) - Nothing to add. Deleted.
v1.4.x (33c2a2c) - Nothing to add. Deleted.
v1.3.x (ee35e72) - Nothing to add. Deleted.
v1.2.x (384c9b4) - Nothing to add. Deleted.
v1.1.x (060de37) - Nothing to add. Deleted.
v1.0.x (30044b0) - Nothing to add. Deleted.
v0.9.x (was at d577502) - PR #2853

[pelson]

Thanks @djkirkham.