Remove redundant management group cipher suite selector writing

Remove redundant management group cipher suite selector writing

Remove the default BIP (AES-128-CMAC) from the code writing the group
management cipher selector into an RSNE since this field is omitted when
the default value is used. This case was unreachable due to the earlier
if statement skipping this switch statement.

Signed-off-by: Rohan Dutta <quic_drohan@quicinc.com>