docs: drop the pkh context roadmap items

The subcommand is gone from the CLI; context survives only as the
internal build backend, so there is no user-facing surface left to
track on the roadmap.

README.md

@@ -121,13 +121,4 @@ Missing features:
 - [ ] 'pkh pull'
   - [ ] Cache the Sources.gz files, to improve speed
   - [ ] Work in an already downloaded package, to git pull and re-fetch orig tar gz
-- [ ] 'pkh context'
-  - [x] Select, add, remove, list contexts
-  - [x] Context-scoped command execution
-  - [ ] Context-scoped deb and test commands
-  - [ ] Per-architecture contexts
-  - [ ] Per-series contexts
-  - [x] ssh contexts
-  - [ ] docker, lxc contexts?
-  - [ ] context push, context pop: context stack