Remove unused argument "isSlice" from transformAssignmentSubscripts()

Remove unused argument "isSlice" from transformAssignmentSubscripts()

Since c7aba7c, the transform method used during parse analysis of a
subcripting construct has moved from transformAssignmentSubscripts() to
the per-type transform method (arrays or arbitrary types) the step that
checks for slicing support, but transformAssignmentSubscripts() has kept
traces of it.  Removing it simplifies the code, so let's clean up all
that.

Author: Zhang Mingli
Reviewed-by: Richard Guo
Discussion: https://postgr.es/m/0d7041ac-c704-48ad-86fd-e05feddf08ed@Spark