Remove If Exist check on migration for mssql because that syntax required SQL server 2016 (#30894)

Fix #30872

We will assume the database is consistent before executing the
migration. So the indexes should exist. Removing `IF EXIST` then is safe
enough.

---------

Co-authored-by: silverwind <me@silverwind.io>
(cherry picked from commit 40de54ece82356b161cdb9cc224ed9004af8ae5d)

Conflicts:
	models/migrations/v1_22/v286.go
	MSSQL is not supported in Forgejo

models/migrations/fixtures/Test_RepositoryFormat/review_state.yml

@@ -1,3 +1,5 @@
 -
   id: 1
+  user_id: 1
+  pull_id: 1
   commit_sha: 19fe5caf872476db265596eaac1dc35ad1c6422d

models/migrations/v1_22/v286_test.go

@@ -19,21 +19,21 @@ func PrepareOldRepository(t *testing.T) (*xorm.Engine, func()) {
 
 	type CommitStatus struct {
 		ID          int64
-		ContextHash string
+		ContextHash string `xorm:"char(40) index"`
 	}
 
 	type RepoArchiver struct {
 		ID       int64
-		RepoID   int64
-		Type     int
-		CommitID string
+		RepoID   int64  `xorm:"index unique(s)"`
+		Type     int    `xorm:"unique(s)"`
+		CommitID string `xorm:"VARCHAR(40) unique(s)"`
 	}
 
 	type ReviewState struct {
 		ID        int64
-		CommitSHA string
-		UserID    int64
-		PullID    int64
+		UserID    int64  `xorm:"NOT NULL UNIQUE(pull_commit_user)"`
+		PullID    int64  `xorm:"NOT NULL INDEX UNIQUE(pull_commit_user) DEFAULT 0"`
+		CommitSHA string `xorm:"NOT NULL VARCHAR(40) UNIQUE(pull_commit_user)"`
 	}
 
 	type Comment struct {