Fix course listings in the admin course.

There are three lines of code changed.

Line 53 of `templates/ContentGenerator/CourseAdmin.html.ep` was deleted.
That line should not exist.  That saves a list of all courses to the
`@courseIDs` variable that is never used.  The `listCourses` method is
called again on line 56 (now 55).

On line 56 (now 55) of that same file, the admin course is omitted from
the listing.  This is how it used to be, and was removed in openwebwork#2295.  I
don't believe that the admin course should be listed on the admin course
listings page.

On line 110 of `templates/ContentGenerator/CourseAdmin/add_course_form.html.ep`
the admin course is kept in the listing.  This is the list of courses
that can be copied.  This was how it was set up in openwebwork#2295, but was
reverted in openwebwork#2290 (probably in merge resolution, or maybe my changes to
that pull request?).  The intent was to allow the admin course to be
copied to change to a new course to use for the admin course.

The first line changed is not controversial, but the second two are up
for discussion.

templates/ContentGenerator/CourseAdmin.html.ep

@@ -50,10 +50,9 @@
 	% }
 	%
 	<h2><%= maketext('Courses') %></h2>
-	% my @courseIDs = listCourses($ce);
 	<ol>
 		% for (sort { lc($a) cmp lc($b) } listCourses($ce)) {
-			% next if $_ eq 'modelCourse';
+			% next if $_ eq $ce->{admin_course_id} || $_ eq 'modelCourse';
 			<li><%= link_to $_ => 'set_list' => { courseID => $_ } =%></li>
 		% }
 	</ol>

templates/ContentGenerator/CourseAdmin/add_course_form.html.ep

@@ -107,7 +107,7 @@
 			. 'select the course and check which components to copy.') =%>
 	</div>
 	<div class="row mb-1">
-		% my @existingCourses = sort { lc($a) cmp lc($b) } grep { $_ ne stash('courseID') } listCourses($ce);
+		% my @existingCourses = sort { lc($a) cmp lc($b) } listCourses($ce);
 		% unshift(@existingCourses, sort { lc($a) cmp lc($b) } @{ $ce->{modelCoursesForCopy} });
 		%
 		<%= label_for copy_from_course => maketext('Copy Components From:'),