haltest: battery: remove registering callback at UnregisterChangedEventP

haltest: battery: remove registering callback at UnregisterChangedEventP

Battery api preserves event callback internally, and doesn't allow
registering an additional callback. Therefore registering callback
in UnregisterChangedEventP generates error as the callback had been
registered at the test right before, the RegisterChangedEventP.
To prevent this, UnregisterChangedEventP reuses callback registered
from RegisterChangedEventP. For the case failing RegisterChangedEventP,
skip the UnregisterChangedEventP.

Change-Id: I92836f2bb68658fd3fd05bfe5755491d2da6d15e
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>