Try this:
USE MyDB
DECLARE @sSql NVARCHAR(4000) SET @sSql = ''
DECLARE @sTypeDesc sysname SET @sTypeDesc = ''
DECLARE @sTableName sysname SET @sTableName = ''
DECLARE @sConstraintName sysname SET @sConstraintName = ''
DECLARE cur CURSOR DYNAMIC FOR
SELECT name,
type_desc,
OBJECT_NAME(parent_object_id)
FROM sys.objects
WHERE is_ms_shipped = 0
AND type_desc LIKE '%_CONSTRAINT'
ORDER BY type_desc, name
WHILE 1 = 1
BEGIN
OPEN cur
FETCH FIRST FROM cur INTO @sConstraintName, @sTypeDesc, @sTableName
IF @@FETCH_STATUS != 0
BEGIN
CLOSE cur
BREAK
END
WHILE @@FETCH_STATUS = 0
BEGIN
BEGIN TRY
SET @sSql = 'ALTER TABLE ' + QUOTENAME(@sTableName) + ' DROP CONSTRAINT ' + QUOTENAME(@sConstraintName)
RAISERROR('%s', 10, 1, @sSql) WITH NOWAIT
EXEC sp_executesql @sSql
END TRY
BEGIN CATCH
DECLARE @ENumber INT SET @ENumber = ISNULL(ERROR_NUMBER(), -1)
DECLARE @ESeverity INT SET @ESeverity = ISNULL(ERROR_SEVERITY(), -1)
DECLARE @estate INT SET @estate = ISNULL(ERROR_STATE(), 0) IF @estate = 0 SET @estate = 42
DECLARE @EProcedure NVARCHAR(126) SET @EProcedure = ISNULL(ERROR_PROCEDURE(), N'{N/A}')
DECLARE @ELine INT SET @ELine = ISNULL(ERROR_LINE(), -1)
DECLARE @EMessageRecv NVARCHAR(2048) SET @EMessageRecv = ISNULL(ERROR_MESSAGE(), N'')
DECLARE @EMessageSent NVARCHAR(440) SET @EMessageSent = N''
IF ERROR_PROCEDURE() IS NOT NULL SET @EMessageSent = N'Error %d, Level %d, State %d, Procedure %s, Line %d, Message: '
SET @EMessageSent = @EMessageSent + ERROR_MESSAGE()
RAISERROR(@EMessageSent, 10 /* @ESeverity */ , @estate, @ENumber, @ESeverity, @estate, @EProcedure, @ELine) WITH LOG
END CATCH
FETCH FIRST FROM cur INTO @sConstraintName, @sTypeDesc, @sTableName
END
CLOSE cur
END
DEALLOCATE cur
GO