Revert r367123: "[llvm] [lit/tests] Replace 'env -u' with more portable construct"

Must be reverted in order to revert r366980, which breaks windows
bots.  See recent D65335 and D65156 comments.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@367626 91177308-0d34-0410-b5e6-96231b3b80d8

utils/lit/tests/lit.cfg

@@ -53,7 +53,7 @@ config.substitutions.append(('%{inputs}', os.path.join(
     config.test_source_root, 'Inputs')))
 config.substitutions.append(('%{lit}',
     "{env} %{{python}} {lit}".format(
-        env="env FILECHECK_OPTS= FILECHECK_DUMP_INPUT_ON_FAILURE=",
+        env="env -u FILECHECK_OPTS -u FILECHECK_DUMP_INPUT_ON_FAILURE",
         lit=os.path.join(lit_path, 'lit.py'))))
 config.substitutions.append(('%{python}', '"%s"' % (sys.executable)))