MAINTAINERS: Remove Michael Clark as a RISC-V Maintainer

Michael is no longer employed by SiFive and does not want to continue
maintianing the RISC-V port.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

MAINTAINERS

@@ -259,7 +259,6 @@ F: include/hw/ppc/
 F: disas/ppc.c
 
 RISC-V
-M: Michael Clark <mjc@sifive.com>
 M: Palmer Dabbelt <palmer@sifive.com>
 M: Alistair Francis <Alistair.Francis@wdc.com>
 M: Sagar Karandikar <sagark@eecs.berkeley.edu>