MAINTAINERS: sifive: drop Yash Shah

Emails to Yash Shah bounce with "The email account that you tried to
reach does not exist.", so drop him from all maintainer entries.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220214082349.162973-1-krzysztof.kozlowski@canonical.com