Remove CSS messages dead code

[rchiodo]

Fixes #10304

[codecov-commenter]

Codecov Report

Merging #10402 (2c936b2) into main (080f35d) will increase coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           main   #10402      +/-   ##
========================================
+ Coverage    64%      69%      +4%     
========================================
  Files       213      479     +266     
  Lines      9506    29451   +19945     
  Branches   1525     5037    +3512     
========================================
+ Hits       6156    20540   +14384     
- Misses     2869     7013    +4144     
- Partials    481     1898    +1417     

Impacted Files	Coverage Δ
src/platform/messageTypes.ts	100% <ø> (ø)
src/webviews/extension-side/variablesView/types.ts	100% <ø> (ø)
src/webviews/extension-side/plotting/types.ts	100% <100%> (ø)
src/webviews/extension-side/webviewHost.ts	74% <100%> (ø)
src/platform/errors/errorHandler.ts	62% <0%> (-1%)	⬇️
...ks/controllers/kernelFilter/kernelFilterService.ts	47% <0%> (ø)
src/kernels/raw/types.ts	100% <0%> (ø)
...rpreter/jupyterInterpreterSelectionCommand.node.ts	100% <0%> (ø)
...coordination/cdnWidgetScriptSourceProvider.base.ts	70% <0%> (ø)
src/kernels/jupyter/jupyterUriProviderWrapper.ts	55% <0%> (ø)
... and 262 more

[DonJayamanne]

isn't forceDark used anywhere. if not we should remove that state prop