CategoryValidation: remove CATEGORY_ prefix from the constants

[0pdd]

The puzzle 927-57fa0b9d from #927 has to be resolved:

mystamps/src/main/java/ru/mystamps/web/feature/category/CategoryValidation.java

Line 22 in 04316e6

// @todo #927 CategoryValidation: remove CATEGORY_ prefix from the constants

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