Remove asymmetric_phy_for_unidirectional_cis feature flag

As the flag have been rollout in 24Q3, the patch is to clean the flag
and the decalration

Bug: 314176433
Fix: 314176433
Flag: com.android.bluetooth.flags.asymmetric_phy_for_unidirectional_cis
Test: m Bluetooth
Change-Id: I12d6f783534db415f20b0ba82947912a2270dcdc