target/sh4: Remove old README.sh4 file

target/sh4: Remove old README.sh4 file

This file didn't have any non-trivial update since it was initially
added in 2006, and looking at the content, it seems incredibly outdated,
saying e.g. "The sh4 target is not ready at all yet for integration in
qemu" or "A sh4 user-mode has also somewhat started but will be worked
on afterwards"... Sounds like nobody is interested in this README file
anymore, so let's simply remove it now.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Yoshinori Sato <ysato@users.sourceforge.jp>
Message-Id: <20220329151955.472306-1-thuth@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>