Fixup 89662a68: remove markers that should not be there

InsetMathDelim does not need markers because the delimiters are
visually enough.

CommandInset (like InsetMathRef) does have cells, but they are hidden
(only for storage). Therefore the default InsetMath::markers() method
mistakenly added markers. In practice, the markers were never active
(since the cursor cannot go into the inset), but some space was
reserved for them.

src/mathed/CommandInset.h

@@ -28,6 +28,8 @@ public:
 	explicit CommandInset(Buffer * buf, docstring const & name,
 		bool needs_math_mode = true);
 	///
+	marker_type marker() const { return NO_MARKER; }
+	///
 	void metrics(MetricsInfo & mi, Dimension & dim) const;
 	///
 	void draw(PainterInfo & pi, int x, int y) const;

src/mathed/InsetMathDelim.h

@@ -32,6 +32,8 @@ public:
 	InsetMathDelim const * asDelimInset() const { return this; }
 	///
 	MathClass mathClass() const { return MC_INNER; }
+	///
+	marker_type marker() const { return NO_MARKER; }
 	/// is it (...)?
 	bool isParenthesis() const;
 	/// is it [...]?