gd: Remove unneeded client check when setting privacy

The check for registered clients can never succeed.

Privacy can only be set once per stack initialization.
No clients may be registered before setting privacy.
Entry ASSERTs ensure the above conditions are validated and true.

Test: gd/cert/run --host
Test: atest --host bluetooth_test_gd
Bug: 171568335
Tag: #refactor
Change-Id: I3fa97028bd104fccae6e37fc1d20196fd6679830