all: Remove miscellaneous vestigial code (#7495)

lib/db/schemaupdater.go

@@ -8,7 +8,6 @@ package db
 
 import (
 	"bytes"
-	"errors"
 	"fmt"
 	"sort"
 	"strings"
@@ -32,8 +31,6 @@ type migration struct {
 	migration           func(prevSchema int) error
 }
 
-var errFolderMissing = errors.New("folder present in global list but missing in keyer index")
-
 type databaseDowngradeError struct {
 	minSyncthingVersion string
 }