Remove unused PERMISSION_PHOTOS_ADD_ONLY macro (#986)

[ibrahim-iqbal]

Fixes #986.

The PERMISSION_PHOTOS_ADD_ONLY macro was declared in PermissionHandlerEnums.h and defined in Package.swift, but nothing #ifdefed against it anywhere in permission_handler_apple/ios/permission_handler_apple/Sources/permission_handler_apple/. The comment on the PERMISSION_PHOTOS define in Package.swift:660-662 already says "the native code compiles photosAddOnly support under PERMISSION_PHOTOS" — that macro is what actually gates the runtime behaviour, so PERMISSION_PHOTOS_ADD_ONLY was just noise.

Changes

• PermissionHandlerEnums.h: drop the #ifndef PERMISSION_PHOTOS_ADD_ONLY / #define / #endif block and its comment.
• Package.swift: drop the corresponding .define("PERMISSION_PHOTOS_ADD_ONLY", ...) entry.
• permission_handler/README.md: the mapping table row for PermissionGroup.photosAddOnly still needs to point contributors at the correct macro. Retarget it to PERMISSION_PHOTOS, which matches the code reality described in the Package.swift comment.

Pre-launch Checklist

• I made sure the project builds. Nothing references the removed macro (grep -R PERMISSION_PHOTOS_ADD_ONLY on the working tree returns no hits), and enabled() only side-effect is populating resolvedMacros for the verbose diagnostic at Package.swift:715-719, which just prints one less row now.
• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I updated pubspec.yaml with an appropriate new version — no runtime behaviour changes on any of the three build paths (CocoaPods, SPM, CMake tests), so no version bump.
• I updated CHANGELOG.md to add a description of the change — dead-code removal with no user-visible effect, happy to add an entry if you'd rather this ship with one.
• I updated/added relevant documentation (the README mapping table).
• I rebased onto main.
• I added new tests to check the change I am making — no observable behaviour to pin.
• I made sure all existing and new tests are passing (no test changes).
• I ran dart format . and committed any changes (no Dart files touched).
• I ran flutter analyze and fixed any errors (no Dart files touched).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.42%. Comparing base (fc60b52) to head (7e4e872).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1567   +/-   ##
=======================================
  Coverage   71.42%   71.42%           
=======================================
  Files           1        1           
  Lines          35       35           
=======================================
  Hits           25       25           
  Misses         10       10           

Flag	Coverage Δ
unittests	71.42% <ø> (ø)

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