Attempt to fix MS SQL drop view for Mat Views facility.

It seems that the current open-coding of the materialized views data
structure is still causing some confusion, so use the same code as in the
MySQL support in the MS SQL parts of the code when trying to fix #950.

See #950.

src/sources/mssql/mssql-schema.lisp

@@ -242,14 +242,10 @@
    (let ((views (remove-if #'null views-alist :key #'cdr)))
      (when views
        (let ((sql
-              (with-output-to-string (sql)
-                (format sql "DROP VIEW ")
-                (loop :for view-definition :in views
-                   :for i :from 0
-                   :do (destructuring-bind (name . def) view-definition
-                         (declare (ignore def))
-                         (format sql
-                                 "~@[, ~]~@[~s.~]~s"
-                                 (not (zerop i)) (car name) (cdr name)))))))
-         (log-message :info "PostgreSQL Source: ~a" sql)
+              (format nil "DROP VIEW ~{~a~^, ~};"
+                      (mapcar (lambda (qname)
+                                (format nil "~@[~s.~]~a"
+                                        (car qname) (cdr qname)))
+                              (mapcar #'car views)))))
+         (log-message :info "MSSQL: ~a" sql)
          (mssql-query sql))))))