Remove incorrect comments from a few test cases.

Author: kevina

src/test/compile-fail/tag-variant-cast-non-nullary.rs

@@ -1,5 +1,4 @@
 //error-pattern: non-scalar cast
-// black and white have the same discriminator value ...
 
 enum non_nullary {
     nullary;

src/test/compile-fail/tag-variant-disr-non-nullary.rs

@@ -1,5 +1,4 @@
 //error-pattern: discriminator values can only be used with a c-like enum
-// black and white have the same discriminator value ...
 
 enum color {
     red = 0xff0000;