[libc++][NFC] Remove a bunch of unused environment variables from the CI configs

GitOrigin-RevId: aaa0dd2f05ff957a171a87e78578dddc59fc49c2
diff --git a/utils/ci/buildkite-pipeline.yml b/utils/ci/buildkite-pipeline.yml
index 36abefe..b8e982b 100644
--- a/utils/ci/buildkite-pipeline.yml
+++ b/utils/ci/buildkite-pipeline.yml
@@ -17,13 +17,6 @@
 # goal being to reduce the load on testers when a commit is known to fail.
 #
 
-env:
-    # LLVM POST-BRANCH bump version
-    # LLVM POST-BRANCH add compiler test for ToT - 1, e.g. "Clang 17"
-    # LLVM RELEASE bump remove compiler ToT - 3, e.g. "Clang 15"
-    LLVM_HEAD_VERSION: "18"   # Used compiler, update POST-BRANCH.
-    GCC_STABLE_VERSION: "13"
-
 definitions:
   _common: &common
     timeout_in_minutes: 120