f_errors: remove unused variant from ColumnSpecParseErrorKind

[muzarski]

In #1135 we removed a check for the table spec across different column specs. Since we wanted to release it in a patch release, the corresponding error variant was not removed (not to introduce API breaking changes).

This commit removes the unused variant.

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• [ ] I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• [ ] I have provided docstrings for the public items that I want to introduce.
• [ ] I have adjusted the documentation in ./docs/source/.
• [ ] I added appropriate Fixes: annotations to PR description.

[github-actions]

cargo semver-checks detected some API incompatibilities in this PR.
Checked commit: 227155a

See the following report for details:

cargo semver-checks output

./scripts/semver-checks.sh --baseline-rev 699e3895ea9fde34fb250e5581207ceee53dd2aa
+ cargo semver-checks -p scylla -p scylla-cql --baseline-rev 699e3895ea9fde34fb250e5581207ceee53dd2aa
     Cloning 699e3895ea9fde34fb250e5581207ceee53dd2aa
    Building scylla v0.15.0 (current)
       Built [  23.812s] (current)
     Parsing scylla v0.15.0 (current)
      Parsed [   0.049s] (current)
    Building scylla v0.15.0 (baseline)
       Built [  22.108s] (baseline)
     Parsing scylla v0.15.0 (baseline)
      Parsed [   0.056s] (baseline)
    Checking scylla v0.15.0 -> v0.15.0 (no change)
     Checked [   0.124s] 127 checks: 127 pass, 0 skip
     Summary no semver update required
    Finished [  47.137s] scylla
    Building scylla-cql v0.4.0 (current)
       Built [  10.845s] (current)
     Parsing scylla-cql v0.4.0 (current)
      Parsed [   0.026s] (current)
    Building scylla-cql v0.4.0 (baseline)
       Built [  10.937s] (baseline)
     Parsing scylla-cql v0.4.0 (baseline)
      Parsed [   0.027s] (baseline)
    Checking scylla-cql v0.4.0 -> v0.4.0 (no change)
     Checked [   0.108s] 127 checks: 126 pass, 1 fail, 0 warn, 0 skip

--- failure enum_variant_missing: pub enum variant removed or renamed ---

Description:
A publicly-visible enum has at least one variant that 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/v0.39.0/src/lints/enum_variant_missing.ron

Failed in:
  variant ColumnSpecParseErrorKind::TableSpecDiffersAcrossColumns, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-699e3895ea9fde34fb250e5581207ceee53dd2aa/4eda4016a3500346a2fafc3e0edc84d78aee3428/scylla-cql/src/frame/frame_errors.rs:430

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [  22.513s] scylla-cql
make: *** [Makefile:61: semver-rev] Error 1