[maintenance]: fix backtrace in xapi-database and drop Backtrace dependency

The backtrace wasn't retrieved before calling the logging function, so
it was potentially lost.
Use Printexc module instead.

Signed-off-by: Edwin Török <edwin.torok@citrix.com>

Author: edwintorok

ocaml/database/dune

@@ -53,7 +53,6 @@
   httpsvr
   unix
   uuid
-  backtrace
   xapi-datamodel
   xapi-log
   (re_export xapi-schema)

ocaml/database/parse_db_conf.ml

@@ -173,8 +173,9 @@ let parse_db_conf s =
     sanity_check !connections ;
     !connections
   with exn ->
+    let bt = Printexc.get_raw_backtrace () in
     error "Database config parse failed: %s" (Printexc.to_string exn) ;
-    Backtrace.reraise exn Cannot_parse_database_config_file
+    Printexc.raise_with_backtrace Cannot_parse_database_config_file bt
 
 let get_db_conf path =
   if Sys.file_exists path then