[SPARK-10715] [ML] Duplicate initialization flag in WeightedLeastSquare

There are duplicate set of initialization flag in `WeightedLeastSquares#add`.
`initialized` is already set in `init(Int)`.

Author: lewuathe <lewuathe@me.com>

Closes #8837 from Lewuathe/duplicate-initialization-flag.

mllib/src/main/scala/org/apache/spark/ml/optim/WeightedLeastSquares.scala

@@ -193,7 +193,6 @@ private[ml] object WeightedLeastSquares {
       val ak = a.size
       if (!initialized) {
         init(ak)
-        initialized = true
       }
       assert(ak == k, s"Dimension mismatch. Expect vectors of size $k but got $ak.")
       count += 1L