Remove stray Jackson 2 files from rest-assured-jackson3 sample (#25129)

RFC3339InstantDeserializer.java and RFC3339JavaTimeModule.java were
accidentally committed in #25112. They are not produced by the generator
for this config (not listed in .openapi-generator/FILES) and use Jackson 2
packages, which breaks compilation of the Jackson 3 sample in the
"Samples Java Client JDK17" workflow.

(cherry picked from commit 2ca276c)

Author: ondrej-simon