intel: Drop silly asserts on mappings present at unmap time.

The intent of these was to catch mismatched map/unmap.  What it
actually did was check whether there was ever a mapping of that type
(including in a previous life of the buffer through the userland BO
cache), not whether they were mismatched.  We don't even actually want
to catch mismatched map/unmap, unless we also do refcounting, since at
one point Mesa would do map/map/use/unmap/unmap.  Just remove this
code instead.