Remove the #externalHide field from the AnnotationLayerBuilder class

[Snuffleupagus]

Prior to PR #20321 the annotationLayer was hidden when there was no regular annotations on the page, which meant that if there were any inferred links (from the textLayer) the annotationLayer needed to be made visible but in such a way that it wouldn't override an explicit hide-call from the PDFPageView class.

With the changes in the aforementioned PR the annotationLayer is now always "visible", and this code can thus be simplified a little bit.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.45%. Comparing base (19046a6) to head (5907d87).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21380      +/-   ##
==========================================
+ Coverage   81.43%   81.45%   +0.01%     
==========================================
  Files         260      260              
  Lines       65843    65839       -4     
==========================================
+ Hits        53617    53626       +9     
+ Misses      12226    12213      -13     

Flag	Coverage Δ
integrationtest	66.73% <ø> (+0.01%)	⬆️
unittest	57.03% <ø> (+0.02%)	⬆️
unittestcli	55.96% <ø> (+<0.01%)	⬆️

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[timvandermeij]

Looks good; thanks!