GH-49632: [C++][R] Remove deprecated old MinGW CMake fixes for AWS

[raulcd]

Rationale for this change

We are using newer R version and MinGW CMAKE_CXX_COMPILER_VERSION is minimum 10.

What changes are included in this PR?

Remove unnecessary code paths for old MinGW on our CMake code.

Are these changes tested?

Yes via CI

Are there any user-facing changes?

No

• GitHub Issue: [C++][R] Remove deprecated checks for old MINGW due to R < 4.2 #49632

[github-actions]

⚠️ GitHub issue #49632 has been automatically assigned in GitHub to PR creator.

[raulcd]

@github-actions crossbow submit -g r

[github-actions]

Revision: e8861d6

Submitted crossbow builds: ursacomputing/crossbow @ actions-c3a14a4ca5

Task	Status
r-binary-packages
r-recheck-most
test-r-alpine-linux-cran
test-r-arrow-backwards-compatibility
test-r-depsource-system
test-r-dev-duckdb
test-r-devdocs
test-r-extra-packages
test-r-fedora-clang
test-r-gcc-11
test-r-gcc-12
test-r-install-local
test-r-install-local-minsizerel
test-r-linux-as-cran
test-r-linux-rchk
test-r-linux-sanitizers
test-r-linux-valgrind
test-r-m1-san
test-r-macos-as-cran
test-r-offline-maximal
test-r-ubuntu-22.04
test-r-versions

[kou]

+1

Good catch!

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 3 benchmarking runs that have been run so far on merge-commit 976d547.

There was 1 benchmark result with an error:

• Commit Run on amd64-m5-4xlarge-linux at 2026-04-02 01:51:00Z
  • dataset-serialize (Python) with dataset=nyctaxi_multi_ipc_s3, format=csv, selectivity=10pc

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 18 possible false positives for unstable benchmarks that are known to sometimes produce them.