chore: Remove unused dependency

[filzrev]

What included in this PR
Remove Docfx.Build.ConceptualDocuments project reference from Docfx.HtmlToPdf.csproj.

Background
Currently Docfx.HtmlToPdf.dll is loaded as plugin DLL at following lines.
Because it transitively referencing Docfx.Plugins project.

docfx/src/Docfx.App/Helpers/DocumentBuilderWrapper.cs

Lines 76 to 85 in fb4fe3f

	if (!IsDocfxPluginAssembly(assemblyFile))
	{
	Logger.LogVerbose($"Skipping non-plugin assembly: {assemblyName}.");
	continue;
	}
	Assembly assembly;
	try
	{
	assembly = Assembly.LoadFrom(assemblyFile);

It seems Docfx.Build.ConceptualDocuments is not used inside projects.
So It can safely removed.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.01% ⚠️

Comparison is base (f698eac) 77.57% compared to head (73eeaf7) 77.57%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9101      +/-   ##
==========================================
- Coverage   77.57%   77.57%   -0.01%     
==========================================
  Files         592      592              
  Lines       24608    24608              
==========================================
- Hits        19090    19089       -1     
- Misses       5518     5519       +1     

see 1 file with indirect coverage changes

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