fix: fix seekable not updating after the first change for live streams

[gesinger]

This removes a check for hasCreatedSourceBuffers that short circuited
onSyncInfoUpdate_. It also adds a comment about how to add the check
in the future, as a simple change of truthiness leads to other issues
in playing back a live stream.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

[codecov]

Codecov Report

Merging #1233 (20d6a25) into main (bfd0ad0) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1233      +/-   ##
==========================================
- Coverage   86.31%   86.30%   -0.02%     
==========================================
  Files          39       39              
  Lines        9796     9796              
  Branches     2278     2278              
==========================================
- Hits         8455     8454       -1     
- Misses       1341     1342       +1     

Impacted Files	Coverage Δ
src/master-playlist-controller.js	94.65% <100.00%> (-0.13%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bfd0ad0...20d6a25. Read the comment docs.