Stop using G_GNUC_CONST in _get_type functions

As per g_type_ensure's documentation it is incorrect to mark _get_type
functions with G_GNUC_CONST since they have side-effects on their first
run.

See https://gitlab.gnome.org/GNOME/glib/-/merge_requests/5223 for more
details.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/819>

src/core/gcal-range-tree.h

@@ -46,7 +46,7 @@ typedef gboolean    (*GcalRangeTraverseFunc)                     (GcalRange
 #define GCAL_TRAVERSE_CONTINUE FALSE;
 #define GCAL_TRAVERSE_STOP     TRUE;
 
-GType                gcal_range_tree_get_type                    (void) G_GNUC_CONST;
+GType                gcal_range_tree_get_type                    (void);
 
 GcalRangeTree*       gcal_range_tree_new                         (void);
 

src/core/gcal-range.h

@@ -78,7 +78,7 @@ typedef enum
   GCAL_RANGE_DATE_ONLY,
 } GcalRangeType;
 
-GType                gcal_range_get_type                         (void) G_GNUC_CONST;
+GType                gcal_range_get_type                         (void);
 
 GcalRange*           gcal_range_new                              (GDateTime          *range_start,
                                                                   GDateTime          *range_end,

src/core/gcal-recurrence.h

@@ -69,7 +69,7 @@ struct _GcalRecurrence
 
 };
 
-GType                gcal_recurrence_get_type                    (void) G_GNUC_CONST;
+GType                gcal_recurrence_get_type                    (void);
 
 GcalRecurrence*      gcal_recurrence_new                         (void);