Cert: Remove dependency to libgrpc++_unsecure

* In newer version of libgrpc++, including both libgrpc++ and
  libgrpc++_unsecure will lead to ODR issue where some symbols
  are defined twice
* Since libgrpc++ is inclusive of libgrpc++_unsecure and we need
  features from libgrpc++, we can just remove the dependency to
  libgrpc++_unsecure to fix this ODR issue.
* When this ODR issue occurs, grpc::InsecureServerCredentials()
  will hang indefinitely, causing the test to timeout.
* Reference: https://github.com/grpc/grpc/issues/24422

Bug: 307877650
Test: m bluetooth_stack_with_facade && gd/cert/run --clean SimpleHalTest
Change-Id: I1f196ec44ab90588891d0a371744b3f816667dd9