Be more precise in our language: call it "mean depth"

As pointed by jwhferguson, I changed "show average depth" to "show
mean depth".

I modified file preferences_graph.ui and changed the text. This bug was
reported on fedora 22. i use ubuntu.layout and justification is nicely
displayed.

The patch also removes a no longer necessary include.

See #922

Signed-off-by: krisalpha <choprakrishan61@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

desktop-widgets/maintab.cpp

@@ -23,7 +23,6 @@
 #include "divelocationmodel.h"
 #include "divesite.h"
 #include "locationinformation.h"
-#include "divesite.h"
 
 #include <QCompleter>
 #include <QSettings>

desktop-widgets/preferences/preferences_graph.ui

@@ -120,7 +120,7 @@
       <item row="6" column="0" colspan="2">
        <widget class="QCheckBox" name="show_average_depth">
         <property name="text">
-         <string>Show average depth</string>
+         <string>Show mean depth</string>
         </property>
        </widget>
       </item>