chore: ignore unnecessary char type cast in GetColumns (#2278)

PostgreSQL有两种char类型：`character`和`"char"`: https://www.postgresql.org/docs/current/datatype-character.html.

`pg_class`里的`relkind`是`"char"`类型，所以`GetColumns`这里应该转成`::"char"`或者和本PR里请求的一样去掉转换。这样对于PostgreSQL以及其他兼容PostgreSQL的数据库容错性更好，在做比较时它们通常都会被隐式转换。
```sql
postgres=# select pg_typeof(relkind), pg_typeof('a'::char), pg_typeof('a'::"char") from pg_class limit 1;
 pg_typeof | pg_typeof | pg_typeof
-----------+-----------+-----------
 "char"    | character | "char"
(1 row)
```

Co-authored-by: August <pin@singularity-data.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-on: https://gitea.com/xorm/xorm/pulls/2278
Co-authored-by: flyingpigge <flyingpigge@noreply.gitea.com>
Co-committed-by: flyingpigge <flyingpigge@noreply.gitea.com>

dialects/postgres.go

@@ -1078,7 +1078,7 @@ FROM pg_attribute f
     LEFT JOIN pg_constraint p ON p.conrelid = c.oid AND f.attnum = ANY (p.conkey)
     LEFT JOIN pg_class AS g ON p.confrelid = g.oid
     LEFT JOIN INFORMATION_SCHEMA.COLUMNS s ON s.column_name=f.attname AND c.relname=s.table_name
-WHERE n.nspname= s.table_schema AND c.relkind = 'r'::char AND c.relname = $1%s AND f.attnum > 0 ORDER BY f.attnum;`
+WHERE n.nspname= s.table_schema AND c.relkind = 'r' AND c.relname = $1%s AND f.attnum > 0 ORDER BY f.attnum;`
 
 	schema := db.getSchema()
 	if schema != "" {