Revert "Pass SWIFT_EXEC to swift-test via bootstrap (#4213)"

This reverts commit 59dc518.

This change was incorrect, `bootstrap` is in fact creating a symlink to the inferior compiler for testing, so overriding `SWIFT_EXEC` is incorrect.

Author: neonichu

Utilities/bootstrap

@@ -361,11 +361,7 @@ def test(args):
 
     note("Testing")
     parse_test_args(args)
-    cmd = [
-        "SWIFT_EXEC=" + args.swiftc_path,
-        "SWIFT_DRIVER_SWIFT_EXEC=" + args.swiftc_path,
-        os.path.join(args.bin_dir, "swift-test")
-    ]
+    cmd = [os.path.join(args.bin_dir, "swift-test")]
     if args.parallel:
         cmd.append("--parallel")
     for arg in args.filter: