re-enable the 0-arg MersenneTwister() constructor

The arguments to `srand(rng, ...)` and `typeof(rng)(...)`
should mirror each-other, in particular
`srand(MersenneTwister(0))` and `MersenneTwister()` should
produce an equivalent object.
Also, `srand(::RandomDevice)` has been added for consistency,
which could be useful in generic code.

Author: rfourquet

base/deprecated.jl

@@ -1307,9 +1307,6 @@ end
      end
 end
 
-# PR #16984
-@deprecate MersenneTwister() MersenneTwister(0)
-
 # #19635
 for fname in (:ones, :zeros)
     @eval @deprecate ($fname)(T::Type, arr) ($fname)(T, size(arr))