Remove extra_navbar from sphinx conf.py

[MFraters]

extra_navbar was removed in executablebooks/sphinx-book-theme#640, and now throws a warning. Since warning are turned in to errors by the settings, we probably need to remove this setting. I will see how the results look like online and see if it need tweaks.

[codecov]

Codecov Report

Merging #481 (715b880) into main (32f081b) will not change coverage.
The diff coverage is n/a.

❗ Current head 715b880 differs from pull request most recent head 35e9107. Consider uploading reports for the commit 35e9107 to get more accurate results

@@           Coverage Diff           @@
##             main     #481   +/-   ##
=======================================
  Coverage   90.85%   90.85%           
=======================================
  Files          91       91           
  Lines        6406     6406           
=======================================
  Hits         5820     5820           
  Misses        586      586           

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[MFraters]

The fix works and the updated theme (which caused the issue) looks good to me.

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.458 ± 0.020 (s=300)	1.458 ± 0.023 (s=320)	-0.4% .. +0.4%
Slab interpolation curved simple none	1.197 ± 0.003 (s=366)	1.197 ± 0.003 (s=389)	-0.1% .. +0.1%
Spherical slab interpolation simple none	1.579 ± 0.084 (s=300)	1.572 ± 0.007 (s=274)	-1.5% .. +0.6%
Slab interpolation simple curved CMS	1.661 ± 0.027 (s=258)	1.665 ± 0.030 (s=286)	-0.2% .. +0.8%
Spherical slab interpolation simple CMS	2.365 ± 0.010 (s=193)	2.364 ± 0.007 (s=190)	-0.2% .. +0.1%
Spherical fault interpolation simple none	1.520 ± 0.007 (s=286)	1.519 ± 0.007 (s=309)	-0.2% .. +0.1%
Cartesian min max surface	4.077 ± 0.027 (s=103)	4.075 ± 0.034 (s=120)	-0.4% .. +0.3%
Spherical min max surface	10.518 ± 0.159 (s=47)	10.477 ± 0.109 (s=41)	-1.3% .. +0.5%