Profile services have const MTU size now

For GAP, the value comes from
`btsock_l2cap_listen_or_connect`. For classic dynamic channel, we use
L2CAP_SDU_LENGTH_MAX for now.

For GATT, we use GATT_MAX_MTU_SIZE, which was used when we send
config request.

Bug: 159815595
Tag: #refactor
Test: compile & verify basic functions working
Change-Id: I93b82c718866f09c4c209311c93b65f2dc74b193