remove the repository feature flags mechanism

[ThomasWaldmann]

Follow-up to #10371, #10372 and #10373: the last content the manifest could carry.

What feature flags were. The manifest's config could hold per-operation (read / check / write / delete) sets of "mandatory" feature names; a borg not supporting one of them refused the operation with MandatoryFeatureUnsupported. The cache config mirrored them as ignored_features / mandatory_features to force a cache rebuild when a version with a different feature set came along.

Why it goes. No feature was ever defined (SUPPORTED_REPO_FEATURES was empty) and nothing ever wrote feature flags, so all of it was dead code: the checks in Manifest.load(), the cache compatibility check / wipe / update, the ManifestItem validation of config["feature_flags"], the compatibility= argument every command decorator had to carry, and a page of docs.

Two commits:

1. Remove the mechanism: manifest checks, cache bookkeeping, item validation, docs (the whole "Feature flags" section of data-structures.rst, the cache config example) and the tests that set an unknown feature. Manifest.Operation and the decorator argument stay as inert placeholders so the commit is green on its own.
2. Remove the placeholders: Manifest.Operation, NO_OPERATION_CHECK, the operations parameter of Manifest.load(), compat_check(), the compatibility= argument of with_repository / with_other_repository, and all call sites (39 files, mechanical).

Kept:

• MandatoryFeatureUnsupported stays defined, never raised, so rc 25 stays reserved.
• The manifest's config dict stays as a general-purpose, preserved-by-all-versions slot; borg currently stores nothing in it. The manifest is now {"version": 2, "archives": {}, "config": {}}.
• Cache config files that still contain the two feature keys load fine (configparser ignores keys nobody reads) and lose them on the next save.

Full suite passes locally (FUSE mount tests deselected: macFUSE is unavailable on this machine after an OS update, they fail identically on master).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.11%. Comparing base (12360cb) to head (f9e112f).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10374   +/-   ##
=======================================
  Coverage   88.10%   88.11%           
=======================================
  Files         103      103           
  Lines       18923    18832   -91     
  Branches     2923     2909   -14     
=======================================
- Hits        16672    16593   -79     
+ Misses       1560     1554    -6     
+ Partials      691      685    -6     

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