L2CAP: Don't allow sending re-config

There is no use case. Remove the support for now. This makes L2CAP state
machine more complicated.

Note that we don't handle re-config request from remote gracefully. For
example, RFCOMM will simply disconnect.

Bug: 159815595
Tag: #refactor
Test: compile & verify basic functions working
Change-Id: I1d2a420e4f904b7e935715f327171d133760eab3