During update, rollback, or MVCC read, we may miscalculate
the number of off-page columns, and thus the size of the
clustered index record. The function btr_push_update_extern_fields()
is mostly redundant, because the off-page columns would also be
moved by row_upd_index_replace_new_col_val(), which is invoked
via row_upd_index_replace_new_col_vals().

btr_push_update_extern_fields(): Remove.

This is based on
mysql/mysql-server@1fa475b85d24de4b9ce2958c0eed738c221fc82c
which refines a fix for a recovery bug fix
mysql/mysql-server@ce0a1e85e24e48b8171f767b44330da635a6ea0a
in MySQL 5.7.5.

No test case was provided by Oracle.
Some of the changed code is being covered by the existing test
innodb.blob-crash.