MAINTAINERS: Remove self from powerpc BPF JIT

Stepping down as I haven't had a chance to look into the powerpc
BPF JIT compilers for a while.

Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210827111905.396145-1-sandipan@linux.ibm.com