tiff: remove wrapper structs from ICC profile API

The TIFF ICC profile functions have struct _openslide_tiff_level and
openslide_t arguments, which in each case only exists for access to one of
the struct's members.  These functions will also be needed internally in
the TIFF decoder for associated images, which don't have the corresponding
abstractions.  We could convert the existing API functions into wrappers
around lower-level internal functions for associated image support, but
the higher-level abstractions aren't really necessary, so just remove them.

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>