METK-144 Remove UserDataContent

[pgeier]

Description

The metkit/codes/DataContent and UserDataContent is removed because it does not relate to eccodes.
It is used in mars-client-cpp and multio. For mars-client-cpp an replacement is provided through https://github.com/ecmwf/mars-client-cpp/pull/60.
For multio a replacement is already in place, although the general usage of eckit::message::Message is revisited.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.07%. Comparing base (931b2ae) to head (d5ee674).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #145      +/-   ##
===========================================
+ Coverage    70.83%   71.07%   +0.24%     
===========================================
  Files          132      130       -2     
  Lines         8188     8160      -28     
  Branches       784      781       -3     
===========================================
  Hits          5800     5800              
+ Misses        2388     2360      -28     

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[danovaro]

lgtm