Remove proactive clean from Phoenix test script

The build system should handle compiler changes correctly. Proactive
clean forces full rebuilds of FFTW/LAPACK from scratch every run,
which is slow and exposes builds to transient filesystem failures
(CMake TryCompile errors on Phoenix scratch).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

.github/workflows/phoenix/test.sh

@@ -1,9 +1,5 @@
 #!/bin/bash
 
-# Clean stale build artifacts from previous CI runs to prevent
-# cross-compiler contamination (e.g. gfortran LAPACK linked by NVHPC)
-./mfc.sh clean
-
 build_opts=""
 if [ "$job_device" = "gpu" ]; then
     build_opts="--gpu"