Remove outdated license text left over from arrow repo

[alamb]

Which issue does this PR close?

Closes #3155

Rationale for this change

I was reviewing code and noticed that LICENSE.txt was a copy/paste from https://github.com/apache/arrow/blob/master/LICENSE.txt when we split out this repo. Thus it contains many incorrect and irrelevant references

What changes are included in this PR?

remove all but the apache license from LICENSE.txt

Are there any user-facing changes?

No (other than the license is now reasonable)

[alamb]

cc @andygrove

[alamb]

I reviewed all these items prior to deleting them to confirm they referred to code that only is in the arrow repository (not arrow-datafusion)

[andygrove]

LGTM. Thanks @alamb.

[codecov-commenter]

Codecov Report

Merging #3154 (ef9fcb7) into master (6509d0d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3154   +/-   ##
=======================================
  Coverage   85.88%   85.88%           
=======================================
  Files         291      291           
  Lines       52720    52720           
=======================================
  Hits        45280    45280           
  Misses       7440     7440           

Impacted Files	Coverage Δ
datafusion/expr/src/logical_plan/plan.rs	77.77% <0.00%> (-0.18%)	⬇️
datafusion/common/src/scalar.rs	84.88% <0.00%> (+0.06%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[ursabot]

Benchmark runs are scheduled for baseline = 4cf0cf0 and contender = 3c0b1c0. 3c0b1c0 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