Remove obsolete DownloadImage() function, from sylabs159

[DrDaveD]

This pulls in

• sylabs/scs-container-library#159

with original PR description of

Removes obsolete (Client).DownloadImage() function
* rename ConcurrentDownloadImage() to DownloadImage()
* rename unexported function ociDownloadBlobPart downloadBlobPart()
* unit test improvements

[codecov]

Codecov Report

Attention: Patch coverage is 48.93617% with 48 lines in your changes are missing coverage. Please review.

Project coverage is 34.70%. Comparing base (acbc137) to head (9fbd68b).

Files	Patch %	Lines
client/push.go	9.75%	37 Missing ⚠️
client/oci.go	42.85%	4 Missing ⚠️
client/pull.go	66.66%	2 Missing and 1 partial ⚠️
client/api.go	83.33%	2 Missing ⚠️
client/version.go	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
- Coverage   35.28%   34.70%   -0.59%     
==========================================
  Files          13       13              
  Lines        1349     1314      -35     
==========================================
- Hits          476      456      -20     
+ Misses        788      781       -7     
+ Partials       85       77       -8     

Flag	Coverage Δ
unittests	34.70% <48.93%> (-0.59%)	⬇️

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