Remove support for Google Spanner database. (#105846)

* Remove support for Google Spanner database.

pkg/storage/unified/sql/sqltemplate/dialect.go

@@ -24,8 +24,6 @@ func DialectForDriver(driverName string) Dialect {
 		return PostgreSQL
 	case "sqlite", "sqlite3":
 		return SQLite
-	case "spanner":
-		return Spanner
 	default:
 		return nil
 	}

pkg/storage/unified/sql/sqltemplate/dialect_spanner.go

@@ -1,32 +0,0 @@
-package sqltemplate
-
-// Spanner is an implementation of Dialect for the Google Spanner database.
-var Spanner = spanner{}
-
-var _ Dialect = Spanner
-
-type spanner struct{}
-
-func (s spanner) DialectName() string {
-	return "spanner"
-}
-
-func (s spanner) Ident(a string) (string, error) {
-	return backtickIdent{}.Ident(a)
-}
-
-func (s spanner) ArgPlaceholder(argNum int) string {
-	return argFmtSQL92.ArgPlaceholder(argNum)
-}
-
-func (s spanner) SelectFor(a ...string) (string, error) {
-	return rowLockingClauseSpanner.SelectFor(a...)
-}
-
-func (spanner) CurrentEpoch() string {
-	return "UNIX_MICROS(CURRENT_TIMESTAMP())"
-}
-
-var rowLockingClauseSpanner = rowLockingClauseMap{
-	SelectForUpdate: SelectForUpdate,
-}