MainWindow: Remove getXPlaceholder() methods
MainWindow: Remove getXPlaceholder() methods

The MainWindow contains getter methods for each placeholder.

They are unused.

Let's remove them.

se-edu/addressbook-level3,damithc/addressbook-level4,CS2103R-Eugene-Peh/addressbook-level4,damithc/addressbook-level4,CS2103R-Eugene-Peh/addressbook-level4,CS2103R-Eugene-Peh/addressbook-level4,damithc/addressbook-level4