Remove SelectableFieldValue from API

[llorllale]

Delete SelectableFieldValue and AssignedField#change() from the API because this flow
adds no real value. Users can just do issue.update().field(field, value); without the extra noise caused by filtering the stream from AssignedField#change().

Here's an example of what's done today (taken from DefaultAssignedFieldIT, which should be renamed to XmlAssignedFieldIT):

final XmlAssignedField field = (XmlAssignedField) issue.fields().stream().findFirst().get();
final FieldValue originalValue = field.value();
final FieldValue newValue = issue.project().fields().stream()
    .filter(f -> field.isSameField(f))
    .findAny().get()
    .values()
    .filter(v -> !v.isEqualTo(originalValue))
    .findAny().get();
final Issue updatedIssue = field.change()
    .filter(v -> v.isEqualTo(newValue))
    .findAny().get()
    .apply();

The final statement can easily be substituted for:

final Issue updatedIssue = issue.update(field, newValue);