doc-rst: pixfmt-nv16m: remove an empty column

The conversion added an empty column (probably, it was used on
DocBook just to increase spacing.

Remove it.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/linux_tv/media/v4l/pixfmt-m420.rst

@@ -129,7 +129,6 @@ Each cell is one byte.
        -  
        -  1
 
-       -  
        -  2
 
        -  
@@ -144,7 +143,6 @@ Each cell is one byte.
        -  
        -  Y
 
-       -  
        -  Y
 
        -  
@@ -156,7 +154,6 @@ Each cell is one byte.
        -  
        -  C
 
-       -  
        -  
        -  
        -  C
@@ -172,7 +169,6 @@ Each cell is one byte.
        -  
        -  Y
 
-       -  
        -  Y
 
        -  
@@ -191,7 +187,6 @@ Each cell is one byte.
        -  
        -  Y
 
-       -  
        -  Y
 
        -  
@@ -203,7 +198,6 @@ Each cell is one byte.
        -  
        -  C
 
-       -  
        -  
        -  
        -  C
@@ -219,7 +213,6 @@ Each cell is one byte.
        -  
        -  Y
 
-       -  
        -  Y
 
        -