Remove unnecessary CHECK in BleAdvertisingManager::Get()

It is causing race conditions. Weak pointer is already taking care of
making sure the instance is valid, even if it was not yet initialized.

Bug: 74863151
Test: compilation
Change-Id: Iddb69f8a38d80044d1bdd7699fad020f710914be
(cherry picked from commit 68ef745a3af2fcdee03db4441016e5b5e0039c74)