Remove declaration of classes that do not exist

Some headers contain
  class Foo;
whereas there is no class Foo.

The list of class statements is given by
  classes=`git grep  '^\(class\|struct\) [a-zA-Z_:]*;' src | sed 's/^.* \(.*\);/\1/'|sort -u`

The ones that are useless are:
  for c in $classes ; do grep -r "\\<$c\\>" src| grep -vq '^[^:]*:\(class\|struct\) [a-zA-Z_:]*;' || echo "$c"; done

src/BufferParams.h

@@ -42,7 +42,6 @@ class Font;
 class HSpace;
 class IndicesList;
 class Language;
-class LatexFeatures;
 class LayoutFile;
 class LayoutFileIndex;
 class Lexer;

src/Paragraph.h

@@ -39,12 +39,10 @@ class DocumentClass;
 class Inset;
 class InsetBibitem;
 class LaTeXFeatures;
-class Inset_code;
 class InsetList;
 class Language;
 class Layout;
 class Font;
-class Font_size;
 class MetricsInfo;
 class OutputParams;
 class PainterInfo;

src/insets/RenderPreview.h

@@ -28,7 +28,6 @@
 namespace lyx {
 
 class Buffer;
-class LyXRC_PreviewStatus;
 class MetricsInfo;
 class PainterInfo;
 

src/mathed/InsetMath.h

@@ -85,11 +85,9 @@ class MaximaStream;
 class MathematicaStream;
 class MathStream;
 class WriteStream;
-class InfoStream;
 
 class MathMacroTemplate;
 class MathMacro;
-class MathPosFinder;
 class Cursor;
 class TextPainter;
 class TextMetricsInfo;