faster generate_subcatalogs

[matthewhanson]

Related Issue(s): #

Description:

The Catalog.generate_subcatalogs function was taking a long time with large catalogs (> several thousand items). Profiling revealed that the Catalog.remove_item was a significant factor. remove_item performs some redundant tasks, and is really not necessary. Instead of removing the item from the original location, when all item links have been moved, those links are removed from the STAC object.

A ~4000 item catalog takes 3 seconds vs 30 seconds to generate the subcatalogs after this change. A 25K item catalog takes about 3 minutes.

PR Checklist:

• Code is formatted (run scripts/format)
• Tests pass (run scripts/test)
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-io]

Codecov Report

Merging #295 (5c16701) into develop (3645114) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #295      +/-   ##
===========================================
- Coverage    93.65%   93.63%   -0.02%     
===========================================
  Files           33       33              
  Lines         4004     4008       +4     
===========================================
+ Hits          3750     3753       +3     
- Misses         254      255       +1     

Impacted Files	Coverage Δ
pystac/catalog.py	95.22% <100.00%> (-0.25%)	⬇️

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