remoteproc/ste: Delete unused driver

Back in July 2014 I asked around what was the intended target
platform for the STE Modem remoteproc driver, so that I could add the
proper hardware dependency to its config option. The answer I got was
that there was no known publicly available hardware needing it and it
was unlikely that there ever would.

So I think it's time to delete this driver to lower the maintenance
burden.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Ohad Ben-Cohen <ohad@wizery.com>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Suman Anna <s-anna@ti.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>