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.