Fix issue 6748

[Lyndon-Li]

Fix issue #6748. Rollback the changes of setting Kopia repo policy, the policy should get into Kopia repo, because some internal processes of Kopia uploader are retrieving the policy from repo. And also remove the snapshot deletion according to Kopia retention policy as the snapshots should be deleted according to Velero's backup retention policy.

[codecov]

Codecov Report

Merging #6765 (d13a233) into main (8d3a675) will increase coverage by 0.13%.
Report is 1 commits behind head on main.
The diff coverage is 89.09%.

@@            Coverage Diff             @@
##             main    #6765      +/-   ##
==========================================
+ Coverage   60.33%   60.46%   +0.13%     
==========================================
  Files         242      242              
  Lines       25992    26018      +26     
==========================================
+ Hits        15681    15731      +50     
+ Misses       9205     9183      -22     
+ Partials     1106     1104       -2     

Files Changed	Coverage Δ
pkg/uploader/kopia/shim.go	74.83% <80.00%> (+6.75%)	⬆️
pkg/uploader/kopia/snapshot.go	81.85% <92.50%> (+1.85%)	⬆️

... and 2 files with indirect coverage changes