Revert "[lit][aix] Always use internal lit shell on AIX (llvm#113355)"

This reverts commit 4583dbc.

Author: abhina-sree

llvm/utils/lit/lit/llvm/config.py

@@ -57,13 +57,6 @@ def __init__(self, lit_config, config):
                 self.lit_config.note("using lit tools: {}".format(path))
                 lit_path_displayed = True
 
-        if platform.system() == "AIX":
-            # Diff on AIX doesn't have all the required features (see
-            # https://github.com/llvm/llvm-project/pull/108871 and
-            # https://github.com/llvm/llvm-project/pull/112997#issuecomment-2429656192)
-            # so always use the internal shell.
-            self.use_lit_shell = True
-
         if platform.system() == "OS/390":
             self.with_environment("_BPXK_AUTOCVT", "ON")
             self.with_environment("_TAG_REDIR_IN", "TXT")