Remove orphans tab feature flag checks

[robertknight]

This PR removes a set of obsolete code paths for supporting the case where the "Orphans" tab was disabled. After this is merged, we can remove the "orphans_tab" feature flag from the service.

There is no time pressure on this one, but I've had it sitting around for a while locally and we should always be getting rid of code that no longer serves a purpose.

[codecov]

Codecov Report

Merging #578 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #578      +/-   ##
==========================================
- Coverage   90.92%   90.92%   -0.01%     
==========================================
  Files         132      132              
  Lines        5347     5334      -13     
  Branches      928      926       -2     
==========================================
- Hits         4862     4850      -12     
+ Misses        485      484       -1

Impacted Files	Coverage Δ
src/sidebar/components/annotation.js	88.94% <ø> (+0.3%)	⬆️
src/sidebar/components/selection-tabs.js	83.33% <ø> (-1.67%)	⬇️
src/sidebar/tabs.js	100% <100%> (ø)	⬆️
src/sidebar/components/sidebar-content.js	89.47% <100%> (-0.14%)	⬇️
src/sidebar/root-thread.js	98.48% <100%> (-0.03%)	⬇️

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[seanh]

Agree, this is a lot of code removal and simplification, it's important to do. Works here and looks good to me