tile: remove set/clear_fixmap APIs

Nothing in the codebase was using them, and as written they took
"unsigned long" as the physical address rather than "phys_addr_t",
which is wrong on tilepro anyway.  Rather than fixing stale APIs,
just remove them; if there's ever demand for them on this platform,
we can put them back.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>