fixmap: Remove unused set_fixmap_offset_io()

The macro set_fixmap_offset_io() was added in commit f774b7d1
("arm64: fixmap: fix missing sub-page offset for earlyprintk") but then
commit 8ef0ed95

 ("arm64: remove arch specific earlyprintk") removed
the file causing the only user to be removed when the two commits were
merged. Since this has never been used again since the v3.15 release
remove it.

Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>