This fixes two problems with MSSQL:
* `ALTER TABLE DROP ... IF EXISTS ...` is only supported in SQL Server >16.
The `IF EXISTS` here is a belt-and-braces and does not need to be present. Therefore
can be dropped. Also stop attempting to drop the indexes as constraints as they're indexes!
* System tables like: `sys.indexes` should be lowercase not uppercase because of collation issues.
Fix#13615
Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>