beckham: Drop support for building twrp

* No this doesn't break twrp support in any way, twrp will still work.
  All this does is remove the makefiles to build twrp using this device
  tree which we don't currently do anyway, so removing for now.

Change-Id: Icd4403e5b73c5aec977a6d5afa31720703eb9b39
Signed-off-by: Joshua Blanchard <joshua.lee.bbg@gmail.com>