config: Remove extraneous BuildConfig setting

[jmooring]

I noticed this in the documentation:

The duplicateResourceFiles key-value pair shouldn't be there. I think this is detritus from the v0.123.0 development cycle (I remember a few iterations related to this functionality). This behavior is now controlled by markup.goldmark.duplicateResourceFiles.

I think we can remove these lines:

hugo/config/commonConfig.go

Lines 101 to 103 in 6b00661

	// When enabled, will duplicate bundled resource files across languages that
	// doesn't have a translated version.
	DuplicateResourceFiles bool