Remove halfviewwidth

- Why I thought I needed this variable in addition to centerx has
  long ago left my memory.

src/doomstat.h

@@ -121,7 +121,6 @@ extern	int 			viewwindowx;
 extern	int 			viewwindowy;
 extern	"C" int 		viewheight;
 extern	"C" int 		viewwidth;
-extern	"C"	int			halfviewwidth;		// [RH] Half view width, for plane drawing
 
 
 

src/r_draw.cpp

@@ -60,7 +60,6 @@ extern	int		ST_Y;
 
 BYTE*			viewimage;
 extern "C" {
-int				halfviewwidth;
 int				ylookup[MAXHEIGHT];
 BYTE			*dc_destorg;
 }

src/r_main.cpp

@@ -351,7 +351,6 @@ void R_SWRSetWindow(int windowSize, int fullWidth, int fullHeight, int stHeight,
 	}
 
 	fuzzviewheight = viewheight - 2;	// Maximum row the fuzzer can draw to
-	halfviewwidth = (viewwidth >> 1) - 1;
 
 	lastcenteryfrac = 1<<30;
 	CenterX = centerx;

src/r_plane.cpp

@@ -1525,7 +1525,7 @@ void R_DrawNormalPlane (visplane_t *pl, double _xscale, double _yscale, fixed_t
 		ystepscale = (DWORD)(-(SDWORD)ystepscale);
 	}
 
-	int x = pl->right - halfviewwidth - 1;
+	int x = pl->right - centerx;
 	planeang += 90;
 	basexfrac = xs_RoundToInt(xscale * planeang.Cos()) + x*xstepscale;
 	baseyfrac = xs_RoundToInt(yscale * -planeang.Sin()) + x*ystepscale;