Fix a bug of binlogging_impossible_mode deprecated warning preventing mysqld to start

Summary:
This is to fix a bug from an upstream commit "Bug#19507567:RENAME VARIABLE
BINLOGGING_IMPOSSIBLE_MODE", in which the deprecated warning of
binlogging_impossible_mode prevents mysqld to start.

Test Plan: Started mysqld successfully after the fix.

Reviewers: santoshb

Reviewed By: santoshb