gatt: Remove invalid MTU setting

Payload size for the transport control block will be set when remote
side sends MTU Exchange response.
We shall not set it before, because it could lead to situation that
invalid MTU is used for some GATT operations which might happen wile MTU
Exchange procedure is ongoing.

Bug: 308403072
Test: connect/disconnect over LE.
Tag: #feature
Change-Id: I60d939865387b09a1bdd3344a03b3398f4908154