random: remove kernel.random.read_wakeup_threshold

It has no effect any more, so remove it.  We can revert this if
there is some user code that expects to be able to set this sysctl.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/a74ed2cf0b5a5451428a246a9239f5bc4e29358f.1577088521.git.luto@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>