MAINTAINERS: Clean up the RISC-V TCG backend section

The e-mail address mjc@sifive.com of Michael is not valid anymore.
Commit 7d04ac3895 removed the entry already from the main
RISC-V section, but apparently forgot to remove it from the TCG
backend section, too.

Fixes: 7d04ac3895
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

MAINTAINERS

@@ -2254,7 +2254,6 @@ F: tcg/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>
 L: qemu-riscv@nongnu.org