build: remove unused swift plugin file and update error message

[kanru]

See also #790 (comment)

Thanks to @ShikiSuen

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.06%. Comparing base (999ee00) to head (036d4c3).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #793   +/-   ##
=======================================
  Coverage   92.06%   92.06%           
=======================================
  Files          37       37           
  Lines        9253     9253           
=======================================
  Hits         8519     8519           
  Misses        734      734           

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[ShikiSuen]

I think we need to add this Provide libchewing as a Swift package with the exposures of C_based APIs only. instead. This ensures that no one expects Swiftified APIs from this package (which brings extra burden to this repo). If anyone wants Swiftified APIs, he / she can make them by himself / herself.

[ShikiSuen]

• Provide libchewing as a Swift package with the exposures of C_based APIs only.

[kanru]

Make sense. For me libchewing refers only to the C API but your edit makes it more clear for people unfamiliar with the history of libchewing.