Remove reduntant variable

Magnifier#mDestroyLock was introduced by below change:
Idf0373ab0d5033d56da0f6f45d7d953f7e796813

Then related code was removed by below change:
Ia4c75b5b997e0ed94d5a3814dd4507a8fffa124d
With this change, Magnifier#mDestroyLock becomes redundant,
useless lock.

This change intends to remove this redundant lock.

Change-Id: Ifa3f83f8953a3d992e26a952d9ac2f3343b0dc9e
Signed-off-by: jiajia tang <tangjiajia@xiaomi.com>