Remove stale references to Cell BE

While reading some code i noticed we're still referencing Cell BE in a couple
parts. This patch removes those.

v2: Update comment in gdb/target.h.

gdb/ChangeLog:

2020-01-02  Luis Machado  <luis.machado@linaro.org>

	* proc-service.c (get_ps_regcache): Remove reference to obsolete
	Cell BE architecture.
	* target.h (struct target_ops) <thread_architecture>: Likewise.

Change-Id: I7a9ccc603b00db22a6275bc5ab69e1417148cb72