gdbserver: remove support for Blackfin

This port has been unmaintained for years and the upstream Linux kernel
does not support this architecture anymore, remove it.

gdbserver/ChangeLog:

	* Makefile.in (SFILES): Remove linux-bfin-low.c.
	* configure.srv: Remove bfin case.
	* linux-bfin-low.cc: Remove.
	* linux-low.cc: Remove BFIN-conditional code.

Change-Id: I846310d15e6386118ec7eabb1b87e647174560fb