ParticipantValidation: remove PARTICIPANT_ prefix from the constants

[0pdd]

The puzzle 927-08aeee4f from #927 has to be resolved:

mystamps/src/main/java/ru/mystamps/web/feature/participant/ParticipantValidation.java

Line 22 in 04316e6

// @todo #927 ParticipantValidation: remove PARTICIPANT_ prefix from the constants

The puzzle was created by Slava Semushin on 08-Jun-19.