This is the Connection List (see chapter 16 in the User's Guide) that replaces the old Logon dialog. It is much more convenient if you define your connections (Tools > Define Connections), but if you do not do so and cancel the dialog every time, it will not be useful at all.
The Connection List logon dialog can be disabled by using the NoConnectionList command-line parameter. For example:
"C:\Program Files\PLSQL Developer\plsqldev.exe" NoConnectionList
You can also set this in the params.ini file in the PL/SQL Developer installation director:
NoConnectionList=1
The Connection List in PL/SQL Developer can be docked or closed. If you subsequently select the "Save Layout" item from the Window menu, this will be the default layout.