kconfig: remove the rootmenu check in menu_add_prop()

This reverts commit ba6ff60d ("kconfig: don't emit warning upon
rootmenu's prompt redefinition").

At that time, rootmenu.prompt was always set first, then it was set
again if a "mainmenu" statement was specified in the Kconfig file.

This is no longer the case since commit 0724a7c3 ("kconfig: Don't
leak main menus during parsing"). Remove the unneeded check.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>