insert deprecation warning for set_graylevel

[jbmohler]

Pursuant to comments in #3683 , this is a deprecation warning for set_graylevel in the backends.

• it is not used (aside from the inheritance chain calls)
• it has never been used according to git history
• it delivers functionality which is trivial to attain other ways

[tacaswell]

@mdboom Do we add an entry to api_changes when we slate stuff for removal? If not, we probably should.

[tacaswell]

👍 On this modulo documentation from me

[mdboom]

Yes -- I think we should an entry into api_changes.rst for this, under a new heading "Changes in 1.5.x". Thanks for doing this, @jbmohler. Removing unused code is always a good thing.

[jbmohler]

yep, the programming days I'm happiest is when my net lines of code is negative!

[tacaswell]

Can you put it in it's own file in the api_changes folder? This is a scheme to reduce doc-conflict caused rebases.

[jbmohler]

@tacaswell that's a good idea. I took the opportunity to clarify my grammar as well.