Big Switch: Remove unnecessary initialization code

Removes a server_timeout parameter that was never set by anything
in the Big Switch plugin and the ML2 driver.

Also eliminates an __init__ method that didn't do anything except
incorrectly log an unset server pool warning before the inheriting
class setup the server pool.

Closes-Bug: #1331249
Change-Id: I77dd22295305b96d4702f1ea0e319dc5f82c71a8
(cherry picked from commit 24a19bfd362996cd9833a167d6eb86fdcf743b75)