[dist] remove distributed version of EnableLinOp

[MarcelKoch]

This PR removes the EnableDistributedLinOp mixin class. Instead, the distributed linops can now directly derive from EnableLinOp. To make this work, the EnablePolymorphicObject now checks if the ConcreteType is derived from DistributedBase, and uses the (exec, comm) constructor in that case. (This is now possible due to the if constexpr in c++17.)

[pratikvn]

Nice! LGTM!

[sonarqubecloud]

Quality Gate passed

Issues
6 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.51%. Comparing base (f296349) to head (df937da).
Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/ginkgo/core/base/polymorphic_object.hpp	60.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1751      +/-   ##
===========================================
- Coverage    89.52%   89.51%   -0.01%     
===========================================
  Files          797      794       -3     
  Lines        65889    65800      -89     
===========================================
- Hits         58989    58904      -85     
+ Misses        6900     6896       -4     

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