highlighter: Remove EmptyDef since it's superseeded by a nil check of SyntaxDef

pkg/highlight/highlighter.go

@@ -67,9 +67,6 @@ func combineLineMatch(src, dst LineMatch) LineMatch {
 // A State represents the region at the end of a line
 type State *region
 
-// EmptyDef is an empty definition.
-var EmptyDef = Def{nil, &rules{}}
-
 // LineStates is an interface for a buffer-like object which can also store the states and matches for every line
 type LineStates interface {
 	LineBytes(n int) []byte