openssh: remove RRECOMMENDS to rng-tools for sshd package

It appears that rngd is not needed as of linux-5.6 and later[1]
and should not be installed by default since the purpose of rngd
is to provide additional trusted sources of entropy.

We did some testing on real hardware, the result seems to support that
we no longer need rngd by default on kernel v5.6 and later.

Testing result as below:

1. observing the crng init stage.
 the "random: crng init done" always available before fs being mounted.

2. generating random number without rngd.
 testing command: dd if=/dev/random of=/dev/null status=progress
   on Marvell CN96xx RDB board, speed almost 20.4 MB/s without block
   on NXP i.mx6q board, speed almost 31.9 MB/s without block
   on qemu x86-64, speed almost 2.6MB/s without block

3. using rngtest command without rngd
 testing command: rngtest -c 1000 </dev/random
   on Marvell CN96xx RDB board:
      rngtest: input channel speed: (min=4.340; avg=135.364; max=146.719)Mibits/s
      rngtest: FIPS tests speed: (min=8.197; avg=69.020; max=72.800)Mibits/s
      rngtest: Program run time: 418771 microseconds

   on NXP i.mx6q board:
      rngtest: input channel speed: (min=96.820; avg=326.769; max=340.598)Mibits/s
      rngtest: FIPS tests speed: (min=15.090; avg=37.543; max=40.324)Mibits/s
      rngtest: Program run time: 570229 microseconds

   on qemu x86-64:
      rngtest: input channel speed: (min=37.769; avg=101.136; max=136.239)Mibits/s
      rngtest: FIPS tests speed: (min=10.288; avg=30.682; max=40.155)Mibits/s
      rngtest: Program run time: 836800 microseconds

4. observing sshd service.
  using "systemctl disable rng-tools" disable service and reboot system.
  system boot up normal, sshd service also start in normal time without
  block.

Reference:
[1] https://github.com/torvalds/linux/commit/30c08efec8884fb106b8e57094baa51bb4c44e32



Signed-off-by: Xiangyu Chen <xiangyu.chen@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>