linker: remove link from external library on unload

When unloading library make sure the soinfo_link to parent
from linked external libraries is removed as well.

Bug: 36104177
Bug: http://b/37433850
Test: manual
Change-Id: I601b54144acecac54744805b38313c46045c54cb
(cherry picked from commit ee398f820700de30941fd82e38bd917e1513cbc6)