spring-projectsGH-924 Remove PaLM API support

spring-projects#1664

  - As a follow up to decommission the PaLM API, the PaLM API support is removed.
  - Remove the model and vector store classes
  - Remove the documentation entries
  - Reference: https://ai.google.dev/palm_docs/deprecation

Resolves spring-projects#924

Signed-off-by: leijendary <[email protected]>

Author: ilayaperumalg