unicode: delete appearance of unused LowerUpper term from comment

If a LowerUpper ever happens, maketables will complain.

Fixes #7002.

LGTM=dave
R=golang-codereviews, dave
CC=golang-codereviews
https://golang.org/cl/59210044

src/pkg/unicode/letter.go

@@ -74,7 +74,7 @@ const (
 
 type d [MaxCase]rune // to make the CaseRanges text shorter
 
-// If the Delta field of a CaseRange is UpperLower or LowerUpper, it means
+// If the Delta field of a CaseRange is UpperLower, it means
 // this CaseRange represents a sequence of the form (say)
 // Upper Lower Upper Lower.
 const (