Remove VisibleForTesting from Processes#getPidOrThrow (#924)

Usage of this package-private method from ProcessHelper
in the same package is a valid usage, so removing the
VisibleForTesting annotation.

Closes #923

src/main/java/org/kiwiproject/base/process/Processes.java

@@ -505,7 +505,6 @@ private static Pair<Long, String> pairFromPgrepLine(String line) {
         return Pair.of(pid, command);
     }
 
-    @VisibleForTesting
     static Long getPidOrThrow(String pidString) {
         try {
             return Long.valueOf(pidString);