adb: delete hellish hodgepodge.

This code was unreachable, since all of the callers were calling
register_socket_transport with foo.c_str() as the serial. Lift this
assumption into the type system by switching from char* to std::string
for the argument type.

Bug: http://b/112147760
Bug: https://www.viva64.com/en/b/0579/
Test: mma
Change-Id: I5a6ee265feee6b83bc933a64d895eed39fce68e7