Bug 23579: Remove default value for MEDIUMTEXT borrower_modifications.changed_fields

Bug 23579: Remove default value for MEDIUMTEXT borrower_modifications.changed_fields

Installation under Ubuntu 18.04 is blocked:
install.pl: DBD::mysql::st execute failed: BLOB, TEXT, GEOMETRY or JSON
column 'changed_fields' can't have a default value at
/usr/local/share/perl5/DBIx/RunSQL.pm line 273.

https://dev.mysql.com/doc/refman/5.7/en/data-type-defaults.html
"""
 The BLOB, TEXT, GEOMETRY, and JSON data types cannot be assigned a
 default value.
"""

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 1d64b1b63df406508bdcde6164b179833f712aeb)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>