btm_io_capabilities_rsp: remove unnecesary null check

To reach this function, device must be bonded or bonding, so it will
have a security record. It's used without null check earlier and later
in this function.

Bug: 333634398
Bug: 377513028
Test: mma -j32;
Flag: com.android.bluetooth.flags.key_missing_classic_device
Change-Id: I05e3da3cda796924eabb0384a40e1ec798584088