gopls/internal/util/frob: use binary.LittleEndian.AppendUint{16,32,64}

Complete a TODO.

Change-Id: I5ef391754d516a9a7672308e1f7e829789f6ac2e

gopls/internal/util/frob/frob.go

@@ -402,38 +402,7 @@ func (r *reader) bytes(n int) []byte {
 type writer struct{ data []byte }
 
 func (w *writer) uint8(v uint8)   { w.data = append(w.data, v) }
-func (w *writer) uint16(v uint16) { w.data = appendUint16(w.data, v) }
-func (w *writer) uint32(v uint32) { w.data = appendUint32(w.data, v) }
-func (w *writer) uint64(v uint64) { w.data = appendUint64(w.data, v) }
+func (w *writer) uint16(v uint16) { w.data = binary.LittleEndian.AppendUint16(w.data, v) }
+func (w *writer) uint32(v uint32) { w.data = binary.LittleEndian.AppendUint32(w.data, v) }
+func (w *writer) uint64(v uint64) { w.data = binary.LittleEndian.AppendUint64(w.data, v) }
 func (w *writer) bytes(v []byte)  { w.data = append(w.data, v...) }
-
-// TODO(adonovan): delete these as in go1.19 they are methods on LittleEndian:
-
-func appendUint16(b []byte, v uint16) []byte {
-	return append(b,
-		byte(v),
-		byte(v>>8),
-	)
-}
-
-func appendUint32(b []byte, v uint32) []byte {
-	return append(b,
-		byte(v),
-		byte(v>>8),
-		byte(v>>16),
-		byte(v>>24),
-	)
-}
-
-func appendUint64(b []byte, v uint64) []byte {
-	return append(b,
-		byte(v),
-		byte(v>>8),
-		byte(v>>16),
-		byte(v>>24),
-		byte(v>>32),
-		byte(v>>40),
-		byte(v>>48),
-		byte(v>>56),
-	)
-}