Remove the useless visibility attribute from this "enum class" declaration.

When compiling under gcc, ANTLR4CPP_PUBLIC macro expands to the following
gcc visibility attribute:
 __attribute__((visibility ("default")))

(when compiling under Windows it expands to the corresponding __declspec
attribute)

This change was introduced in commit 8ff852640a

Although the attribute makes perfect sense when applied to a "class"
declaration, it makes no sense (has no effect) when applied to an
"enum class" declaration. I assume that doing so was unintentional; that
when the change was introduced it was it was added mechanically to all
"class XXX" instances in the source code, a process which accidentally
picked up one "enum class XXX" instance.

Although it has no effect on the object code, it leads to the following
warning when compiling under gcc:
/usr/local/include/antlr4-runtime/atn/PredictionMode.h:18:31: error: type attributes ignored after type is already defined [-Werror=attributes]
   enum class ANTLR4CPP_PUBLIC PredictionMode {

This is a problem for people who would like their builds to be warning-free.

Happily, this declaration can be safely removed. The "enum class" construct
(just like with regular enum) does not cause any linker symbols to be
emitted. So having a linker attribute on the type does not actually have any
effect. It can therefore be safely removed.

runtime/Cpp/runtime/src/atn/PredictionMode.h

@@ -15,7 +15,7 @@ namespace atn {
    * utility methods for analyzing configuration sets for conflicts and/or
    * ambiguities.
    */
-  enum class ANTLR4CPP_PUBLIC PredictionMode {
+  enum class PredictionMode {
     /**
      * The SLL(*) prediction mode. This prediction mode ignores the current
      * parser context when making predictions. This is the fastest prediction