asm-generic: Remove empty asm/unistd.h

Nothing is left in asm/unistd.h except for the redirect to
uapi/asm/unistd.h, so removing the file simply leads to that one being
used directly.  The linux/export.h inclusion is a leftover from commit
e1b5bb6d ("consolidate cond_syscall and SYSCALL_ALIAS declarations")
and should not be used anyway.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>