PersonCard: remove #equals(object)

This method implements a very weak notion of equivalence, where as long
as the PersonCard's person and id are equal, the object is equal.
However, such a weak notion of equivalence goes against the spirit of
the UI architecture, which states that all UiParts represent a distinct
part of the UI, and more importantly, wraps a subgraph of the JavaFX
scene graph (accessible via UiPart#getRoot()). As such, just because two
PersonCards A and B happen to contain the same ID and person value,
doesn't mean they are "equal", because they may be holding different
JavaFX scene graph objects. Any JavaFX-related operations on PersonCard
A may give a different result to PersonCard B.

Furthermore, this implementation of PersonCard#equals(Object) isn't
actually used, not even by test code. It was introduced in baa5988
(PersonListPanelHandle: Update methods, 2017-07-14) so that one could
do the following to assert that a certain PersonCard was selected:

    private void assertCardSelected(Index index) {
        PersonCard selectedCard = getPersonListPanel().getSelectedCard().get();
        assertEquals(getPersonListPanel().getCard(index.getZeroBased()), selectedCard);
    }

where the assertEquals() would then call PersonCard#equals(Object).

However, since 04a675f (PersonListPanelHandle: Update
getSelectedCard(), 2017-07-20) this pattern of code has been (rightly)
removed, with test code preferring to compare what is actually displayed
on the PersonCard, rather than using PersonCard#equals(Object):

    /**
     * Asserts that {@code actualCard} displays the same values as {@code expectedCard}.
     */
    public static void assertCardEquals(PersonCardHandle expectedCard, PersonCardHandle actualCard) {
        assertEquals(expectedCard.getId(), actualCard.getId());
        assertEquals(expectedCard.getAddress(), actualCard.getAddress());
        assertEquals(expectedCard.getEmail(), actualCard.getEmail());
        assertEquals(expectedCard.getName(), actualCard.getName());
        assertEquals(expectedCard.getPhone(), actualCard.getPhone());
        assertEquals(expectedCard.getTags(), actualCard.getTags());
    }

PersonCard#equals(Object) was, however, not removed then. So let's
remove it now.

src/main/java/seedu/address/ui/PersonCard.java

@@ -56,20 +56,4 @@ public PersonCard(Person person, int displayedIndex) {
                 .sorted(Comparator.comparing(tag -> tag.tagName))
                 .forEach(tag -> tags.getChildren().add(new Label(tag.tagName)));
     }
-
-    @Override
-    public boolean equals(Object other) {
-        if (other == this) {
-            return true;
-        }
-
-        // instanceof handles nulls
-        if (!(other instanceof PersonCard)) {
-            return false;
-        }
-
-        PersonCard card = (PersonCard) other;
-        return id.getText().equals(card.id.getText())
-                && person.equals(card.person);
-    }
 }