Revert following bug fix:

Bug#20685029: SLAVE IO THREAD SHOULD STOP WHEN DISK IS
FULL
Bug#21753696: MAKE SHOW SLAVE STATUS NON BLOCKING IF IO
THREAD WAITS FOR DISK SPACE

This fix results in a deadlock between slave IO thread
and SQL thread.

(cherry picked from commit e3fea6c6dbb36c6ab21c4ab777224560e9608b53)