MAINTAINERS: Remove Gregory and Brian for ARCH_BRCMSTB

The last time Gregory and Brian did a review was sometime around 2015,
since then, they have not been active for ARCH_BRCMSTB changes.
Following the position of other maintainers and Harald Welte's position
here:

[1] http://laforge.gnumonks.org/blog/20180307-mchardy-gpl/



remove both of them.

Acked-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>