[diff.expr] Remove commentary about good practice in C (#8356)

Author: Keith-S-Thompson

source/compatibility.tex

@@ -2996,10 +2996,7 @@
 \end{example}
 
 \howwide
-This is fairly widely used but it is good
-programming practice to add the cast when assigning pointer-to-void to pointer-to-object.
-Some C translators will give a warning
-if the cast is not used.
+Common.
 
 \diffref{expr.arith.conv}
 \change