MINOR: remove out-of-date is_dictionary checks from binary_rule.rs

[kmitchener]

Which issue does this PR close?

Closes #.

Rationale for this change

These reference an old PR that was resolved in arrow-rs months ago. Tests still pass without these checks.

What changes are included in this PR?

Are there any user-facing changes?

[kmitchener]

this is_dictionary() call would never be true anyway due to the checks above. probably copy/paste duplication?

[alamb]

Thanks @kmitchener

cc @tustvold

I restarted the failing CI job (mac) as it seems to be unrelated to this PR and more of an infrastructure problem

[alamb]

Thanks again @kmitchener

[ursabot]

Benchmark runs are scheduled for baseline = ff718d0 and contender = cec6395. cec6395 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java