box: drop sql_defer_foreign_keys session setting

This patch removes sql_defer_foreign_keys from the _session_setting
system space, since foreign keys can no longer be deferred.

Part of #6986

@TarantoolBot document
Title: Deletion of "sql_defer_foreign_keys" session setting.

Because "constraint check time" is no longer supported by foreign keys,
the session setting "sql_defer_foreign_keys" has no meaning and is
discarded.