Drop --exec_constraints from rbe-configs/generate.py

[hvadehra]

rbe_configs_gen does not and AFAICT never did expose such a flag.

In fact, the flag had a typo from when it was introduced till cfad46d several months later which makes me think this script was either never used, or if it was, it was used with a custom build of rbe_configs_gen that does expose that flag.

[coeuvre]

I used a custom build of rbe_configs_gen. The PR for adding the flag is bazelbuild/bazel-toolchains#968 but I never managed to make it merged. Maybe I should.

[hvadehra]

I used a custom build of rbe_configs_gen. The PR for adding the flag is bazelbuild/bazel-toolchains#968 but I never managed to make it merged. Maybe I should.

AFAICT, the exec_constraints are the same as the defaults in rbe_configs_gen. Only diff is @bazel_tools//tools/cpp:gcc vs @bazel_tools//tools/cpp:clang which are anyways identical.

[coeuvre]

Did we change how C++ toolchain auto-detection works in Bazel?

More specifically, the requirement was we want this configuration to use gcc instead clang in the docker image.

[hvadehra]

My bad, please ignore what I said, I misread the BUILD.tools file.