PERF: Change the parent class of RegularStepGradDescentv4 optimizer

In the previous inheritance hierarchy the RSGDescentOptimizerv4
was inherited from the itkGradientDescentOptimizerBasev4 class. Now regular step
gradient descent filter is inherited from GradientDescentOptimizerv4, so
it can use the learning rate estimation features of gradient descent optimizer.

The new hierarchy also helped us to remove the duplicated codes.

Change-Id: I8ea7bea97978bcd4356597fc7869a7ddb120e3eb
Merge topic 'RSGDOptv4_r'

98dbea9c PERF: Change the parent class of RegularStepGradDescentv4 optimizer