Remove BAZEL_USE_CPP_ONLY_TOOLCHAIN from BCR presubmit

This is enabled on Windows and MacOS automatically in `.bazelrc.common`.

Author: avdv

.bcr/presubmit.yml

@@ -32,7 +32,6 @@ tasks:
     - 'powershell -Command "Invoke-WebRequest -Uri https://hackage.haskell.org/root.json -OutFile out.json"'
     build_flags:
       - '--incompatible_enable_cc_toolchain_resolution'
-      - '--repo_env=BAZEL_USE_CPP_ONLY_TOOLCHAIN=1'
     build_targets:
     - '@rules_haskell//haskell/...'
     - '@rules_haskell//tools/...'