fix: don't set MPICH_GPU_SUPPORT_ENABLED for frontier_amd

AMD flang on Frontier uses AFAR/ROCm GPU-aware MPI, not Cray's GTL.
The module loading script explicitly unsets MPICH_GPU_SUPPORT_ENABLED.
Setting it via gpu_env causes 'GTL library is not linked' errors.
Only frontier.mako (CCE) needs this variable.

Author: Spencer Bryngelson

toolchain/templates/frontier_amd.mako

@@ -5,8 +5,7 @@
 mpi_config = {
     "binary":    "srun",
     "flags":     ["--unbuffered", "--cpus-per-task", "7"],
-    "env":       {"MPICH_GPU_SUPPORT_ENABLED": "0"},
-    "gpu_env":   {"MPICH_GPU_SUPPORT_ENABLED": "1"},
+    "env":       {},
     "gpu_flags": [],
 }
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