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>

MAINTAINERS

@@ -3242,8 +3242,6 @@ S:	Maintained
 F:	drivers/usb/gadget/udc/bcm63xx_udc.*
 
 BROADCOM BCM7XXX ARM ARCHITECTURE
-M:	Brian Norris <computersforpeace@gmail.com>
-M:	Gregory Fong <gregory.0xf0@gmail.com>
 M:	Florian Fainelli <f.fainelli@gmail.com>
 M:	bcm-kernel-feedback-list@broadcom.com
 L:	linux-arm-kernel@lists.infradead.org (moderated for non-subscribers)