kconfig: remove redundant assignment prompt = prompt

Semi-automatic removing of localization macros changed the line
from "prompt = _(prompt);" to "prompt = prompt;". Drop the
reduntand assignment.

Fixes: 694c49a7 ("kconfig: drop localization support")
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>