Before MDEV-5800 in MariaDB 10.2.2, InnoDB knew nothing about
virtual columns. But, they would be present in the TABLE_SHARE.

It appears that the MDEV-12936 fix only worked in the special case
when the virtual columns are the last columns in a table definition.

mysql_fields(): Remove.

build_template_needs_field(): Omit virtual columns if the
InnoDB table definition is known to not contain them, based
on the .frm file version.

build_template_field(): Clean up some code.

ha_innobase::build_template(): Remove redundant computations.
Loop over all MariaDB columns. Skip virtual columns if
InnoDB does not know about them.

calc_row_difference(): Skip not-known-to-InnoDB virtual columns.

wsrep_calc_row_hash(): Skip all virtual columns. This would
fix an out-of-bounds access to dict_table_t::cols[]. Also,
remove some redundant computations and variables.

create_table_check_doc_id_col(): Assert that the .frm file
for a newly created table will be in a format where InnoDB
knows about virtual columns.