radix tree test suite: delete unused rcupdate.c

This file was used to implement call_rcu() before liburcu implemented
that function.  It hasn't even been compiled since before the test suite
was added to the kernel.  Remove it to reduce confusion.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Matthew Wilcox <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Konstantin Khlebnikov <[email protected]>
Cc: Ross Zwisler <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>