cleanup: remove duplicated NSEC_PER_SEC macros

This switches everything to using a single 64-bit constant for NSEC_PER_SEC
that ensure we're doing 64-bit math everywhere.

src/libsysprof-ui/sysprof-cell-renderer-duration.c

@@ -26,8 +26,6 @@
 #include "sysprof-ui-private.h"
 #include "sysprof-zoom-manager.h"
 
-#define NSEC_PER_SEC (G_USEC_PER_SEC * 1000L)
-
 typedef struct
 {
   gint64 capture_begin_time;