MAINTAINERS: remove defunct spear mailing list

It looks like the email address for this mailing list doesn't exist anymore:
<spear-devel@list.st.com>: host mxb-00178001.gslb.pphosted.com[91.207.212.93] said:
550 5.1.1 User Unknown (in reply to RCPT TO command)

Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Jiri Slaby <jslaby@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>