MDEV-5816: Stored programs: validation of stored program statements

This is the prerequisite patch to remove the data member
sp_head::m_trg_table_fields and the method is_fields_updated_in_trigger
that used it but is not called anywhere in the source code.

The commit 5f1f2fc0 introduced the
data member sp_head::m_trg_table_fields and the method
Table_triggers_list::is_fields_updated_in_trigger() that used this data member.

The method Table_triggers_list::is_fields_updated_in_trigger() was invoked
by the method partition_info::can_prune_insert() also introduced by
the same commit 5f1f2fc0

The method partition_info::can_prune_insert() is not called anywhere
in the code and later these  methods were removed from the source code
but the data member sp_head::m_trg_table_fields wasn't.
So, remove the data member sp_head::m_trg_table_fields and declaration of
the method is_fields_updated_in_trigger() for purpose of code cleaning up.

sql/sp_head.h

@@ -323,15 +323,6 @@ class sp_head :private Query_arena,
   */
   Security_context m_security_ctx;
 
-  /**
-    List of all items (Item_trigger_field objects) representing fields in
-    old/new version of row in trigger. We use this list for checking whenever
-    all such fields are valid at trigger creation time and for binding these
-    fields to TABLE object at table open (although for latter pointer to table
-    being opened is probably enough).
-  */
-  SQL_I_List<Item_trigger_field> m_trg_table_fields;
-
 protected:
   sp_head(MEM_ROOT *mem_root, sp_package *parent, const Sp_handler *handler,
           enum_sp_aggregate_type agg_type);

sql/sql_trigger.h

@@ -145,7 +145,6 @@ class Trigger :public Sql_alloc
   trg_action_time_type action_time;
   uint action_order;
 
-  bool is_fields_updated_in_trigger(MY_BITMAP *used_fields);
   void get_trigger_info(LEX_CSTRING *stmt, LEX_CSTRING *body,
                         LEX_STRING *definer);
   /* Functions executed over each active trigger */