src/poky:utils: Drop obsolete oe_machinstall function

This function does something similar to the bitbake fetcher file search
paths and is an old remnant of how this could have once been handled. There
are no users in OE-Core and we'd always prefer the fetcher to handle this now
as it handles signatures correctly. The direct OVERRIDES reference would
likely break any user of the function. Remove it to be safe and ensure it
isn't being used anywhere.

(From OE-Core rev: d49ba0243e3d28672d16cd02753eb7e85d91bbab)

Signed-off-by: Richard Purdie <[email protected]>