Remove redundant environment variable XLA_FLAGS.

If the flag is not set, the script saved_model_aot_compile.py in tensorflow will
default it to the correct value. However, in TF 2.5, the way the value is set in
TensorFlowCompile.cmake file triggers a build error.

Reviewed By: mtrofin

Differential Revision: https://reviews.llvm.org/D103972