arch/um: remove depends on CONFIG_EXPERIMENTAL

The CONFIG_EXPERIMENTAL config item has not carried much meaning for a
while now and is almost always enabled by default. As agreed during the
Linux kernel summit, remove it from any "depends on" lines in Kconfigs.

CC: Jeff Dike <jdike@addtoit.com>
CC: Richard Weinberger <richard@nod.at>
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Richard Weinberger <richard@nod.at>