MAINTAINERS: Drop obsolete entries from Samsung sxgbe ethernet driver

The emails to ks.giri@samsung.com and vipul.pandya@samsung.com bounce
with 550 error code:

    host mailin.samsung.com[203.254.224.12] said: 550
    5.1.1 Recipient address rejected: User unknown (in reply to RCPT TO
    command)"

Drop Girish K S and Vipul Pandya from sxgbe maintainers entry.

Cc: Byungho An <bh74.an@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>