Revert addition of DYLIB-LLVM-AOT as a default backend (#2702)

This is a partial reversion of https://github.com/google/iree/pull/2639.
The change to Bazel build scripts was preserved to keep the uniform
environment between Bazel builds running with RBE.

Part of https://github.com/google/iree/issues/2645
diff --git a/CMakeLists.txt b/CMakeLists.txt
index c4c5504..b761338 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -116,7 +116,7 @@
 
 # List of all target backends to be built by default:
 set(IREE_ALL_TARGET_BACKENDS
-  DYLIB-LLVM-AOT
+  # TODO(#2645): Add DYLIB-LLVM-AOT when it doesn't require an env var
   LLVM-IR
   Vulkan-SPIRV
   VMLA