When type of the settable global variable innodb_change_buffering was
changed from string to ENUM in MDEV-12218, the "shadow" variable ibuf_use
stopped being updated as a result of SET GLOBAL innodb_change_buffering.
Only on InnoDB startup, the parameter innodb_change_buffering would
take effect.

ibuf_use: Remove, and use the global variable innodb_change_buffering.