Remove dead `ValidatorGenerator`/`ApplyValidator` (unreachable, superseded by `ValidatorRegistry`)

[alexander-yevsyukov]

Summary

ValidatorGenerator and its nested ApplyValidator
(java/src/main/kotlin/io/spine/tools/validation/java/generate/ValidatorGenerator.kt,
~49 lines) appear to be dead code — never instantiated anywhere in the
repository.

Found while raising test coverage in #318: the class shows as 100% uncovered
because it is unreachable, not because it lacks tests.

Evidence

• A repo-wide search for ValidatorGenerator( finds no instantiation — only
  self-references within its own file.
• The nested-message validation it was designed to generate is instead handled
  at runtime by io.spine.validation.ValidatorRegistry.validate(this),
  invoked from ValidationCodeInjector.validateMethodBody
  (java/src/main/kotlin/io/spine/tools/validation/java/generate/ValidationCodeInjector.kt,
  ~line 153). So ValidatorGenerator/ApplyValidator is a superseded
  compile-time approach.

Proposed action

1. Verify the classes are genuinely unused — check reflection / ServiceLoader
   / DI wiring too, not just direct constructor calls.
2. If confirmed dead, remove ValidatorGenerator.kt along with any now-unused
   imports/helpers it pulled in.
3. Build :java and run the validation test suites to confirm nothing breaks.

This is a production-code change, so it is intentionally not part of the
coverage-only PR #318 — it needs its own review.

Filed as a follow-up from #318.