move POBackupLearner out of attic

[mb706]

Low priority, if this is ever shown to have benefits over the mlr3-internal functionality.

This reverts commit 58fb1d0.

[codecov-io]

Codecov Report

Merging #204 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #204      +/-   ##
==========================================
+ Coverage   95.36%   95.39%   +0.03%     
==========================================
  Files          36       37       +1     
  Lines        1382     1413      +31     
==========================================
+ Hits         1318     1348      +30     
- Misses         64       65       +1

Impacted Files	Coverage Δ
R/PipeOpBackupLearner.R	96.77% <ø> (ø)

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