fix: Return value instead of SettingsAttribute object when using pop method

[Laerte]

As reported on #5959 (comment), the implementation of pop method is returning SettingsAttribute object instead of the value itself which break interface. Mea culpa, sorry! 😅

[codecov]

Codecov Report

Merging #5963 (a252597) into master (5105742) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

❗ Current head a252597 differs from pull request most recent head 90cfbd1. Consider uploading reports for the commit 90cfbd1 to get more accurate results

@@            Coverage Diff             @@
##           master    #5963      +/-   ##
==========================================
- Coverage   89.19%   89.12%   -0.08%     
==========================================
  Files         162      162              
  Lines       11287    11287              
  Branches     1833     1833              
==========================================
- Hits        10067    10059       -8     
- Misses        928      934       +6     
- Partials      292      294       +2     

Impacted Files	Coverage Δ
scrapy/settings/__init__.py	98.34% <100.00%> (ø)

... and 1 file with indirect coverage changes

[Gallaecio]

Thanks!