MAINTAINERS: Karthikeyan Ramasubramanian is MIA

A mail just bounced back with "user unknown":

550 5.1.1 <kramasub@codeaurora.org> User doesn't exist

I also couldn't find a more recent address in git history. So, remove
this stale entry.

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>