Add the default methods of j.u.Iterator default methods

[LeeTibbert]

• This PR provides the missing remove() default method. This method is used
  by the Scala.js TestMainBase hierarchy. Having this method,
  along with previous PR Add j.l.Iterable.forEach #1934 allows JUnit CollectionsTest.scala
  and others to link and execute, with the expected zero tests run.

• For completeness, this PR also provides the forEachRemaining()
  default method. It is not used by the TestMainBase hierarchy
  and could be deferred to a separate PR. The overall project cost of
  including it here seemed to be less than the cost of a separate PR.

• The effective change is in Iterator.scala. Because it now provides
  a default message, a number of other files need overrides. There
  should be no functional change.

• There is no direct corresponding JUnit test for remove() because
  of circularity. This method is used by the environment which would test any
  standalone file. These changes will be extensively exercised
  by that test environment and defects will become obvious.

• forEachRemaining() has a test in Scala.js IteratorTest.scala. That
  change in this PR Will become publicly tested when the current work to port
  the TestMainBase environment finishes. Until then, it is being heavily
  exercised in my porting environment.

Documentation:

• None required

Testing:

Safety -

• Built and tested ("test-all") in debug mode using sbt 1.3.13 on
  X86_64 only . All tests pass.

Efficacy -

• Will be shown when the port of the Scala.js TestMainBase environment
  to Scala Native has been accomplished.