chore(expr-common): remove deprecated Signature get_possible_types (Closes #23080 - partial)

[Dodothereal]

Removes Signature::get_possible_types (deprecated since 46.0.0 with replacement get_example_types). Pure 5-line deletion; the in-tree unit test already calls get_example_types directly, so no test updates needed. Closes #23080 (partial - fifth in housekeeping series after #23129, #23131, #23132, #23134). AI assistance: used an AI coding assistant; verified via repo-wide grep that the function is unused outside tests.

[Dodothereal]

Re-pinging: this PR removes Signature::get_possible_types (part of the #23080 deprecation sweep). 5-line removal in expr-common; checks already passed on the parts-3 PR (#23132). Happy to re-push or rebase if anything's needed.

[Dodothereal]

@alamb (and any other reviewer) — gentle ping: this is the 5th and last non-experimental removal in the #23080 sweep; the first three (#23129, #23131, #23132) landed today, #23134 was just opened for the 4th. Branch is up to date with upstream/main; cargo +nightly semver-checks is happy on the previous PRs. Happy to address any feedback.

[Dodothereal]

Friendly nudge — same pattern as #23134 (5-line pure-removal, zero callers). Ready when a maintainer can approve CI / merge.

[github-actions]

Thank you for opening this pull request!

Reviewer note: cargo-semver-checks reported the current version number is not SemVer-compatible with the changes in this pull request (compared against the base branch).

Details

     Cloning apache/main
    Building datafusion-expr-common v54.0.0 (current)
       Built [  24.743s] (current)
     Parsing datafusion-expr-common v54.0.0 (current)
      Parsed [   0.020s] (current)
    Building datafusion-expr-common v54.0.0 (baseline)
       Built [  17.786s] (baseline)
     Parsing datafusion-expr-common v54.0.0 (baseline)
      Parsed [   0.019s] (baseline)
    Checking datafusion-expr-common v54.0.0 -> v54.0.0 (no change; assume patch)
     Checked [   0.340s] 223 checks: 222 pass, 1 fail, 0 warn, 30 skip

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.48.0/src/lints/inherent_method_missing.ron

Failed in:
  TypeSignature::get_possible_types, previously in file /home/runner/work/datafusion/datafusion/target/semver-checks/git-apache_main/668f342273aa5c7408eced53df4a09f150f4afad/datafusion/expr-common/src/signature.rs:884

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [  44.041s] datafusion-expr-common