xtensa: remove hifi_mini sim target

The core build in the SDK does not have a timer, making it impossible
to use this core with most of the sanity checks. We are working on
getting a special package of cores for Zephyr which meet our build
requirments; until then, remove this core as it doesn't build.

Change-Id: I3fa201f3c6b5724501e8cb1e1b8ba631436ebc23
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>