[DFSan] Remove use of setarch in dfsan test.

Use of setarch Was added by
https://github.com/llvm/llvm-project/commit/f93c2b64ed381bc4bc52f4b8539026f1597f574d

Running the test now it doesn't seem necessary because:

1) Explicitly only x86_64 is supported for dfsan.

2) https://reviews.llvm.org/D111522 makes it less flakey.

Differential Revision: https://reviews.llvm.org/D121439