MAINTAINERS: Update brcmstb entry

We have not heard back from Marc in a while, since we last asked him to
ack/nack this change here: https://lkml.org/lkml/2015/1/14/1004



This was back in January, we are in the 3.21^W4.1 merge window now, so,
as outlined in this email thread, remove him from the MAINTAINERS list
now.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>