Revert "[llvm] Use lit internal shell by default"

This reverts commit 2581354.

This is causing test failures on Darwin.

https://lab.llvm.org/buildbot/#/builders/190/builds/27101

Author: boomanaiden154

llvm/test/lit.cfg.py

@@ -17,17 +17,6 @@
 # name: The name of this test suite.
 config.name = "LLVM"
 
-# TODO: Consolidate the logic for turning on the internal shell by default for all LLVM test suites.
-# See https://github.com/llvm/llvm-project/issues/106636 for more details.
-#
-# We prefer the lit internal shell which provides a better user experience on failures
-# and is faster unless the user explicitly disables it with LIT_USE_INTERNAL_SHELL=0
-# env var.
-use_lit_shell = True
-lit_shell_env = os.environ.get("LIT_USE_INTERNAL_SHELL")
-if lit_shell_env:
-    use_lit_shell = lit.util.pythonize_bool(lit_shell_env)
-
 # testFormat: The test format to use to interpret tests.
 extra_substitutions = extra_substitutions = (
     [
@@ -37,7 +26,9 @@
     if config.enable_profcheck
     else []
 )
-config.test_format = lit.formats.ShTest(not use_lit_shell, extra_substitutions)
+config.test_format = lit.formats.ShTest(
+    not llvm_config.use_lit_shell, extra_substitutions
+)
 
 # suffixes: A list of file extensions to treat as test files. This is overriden
 # by individual lit.local.cfg files in the test subdirectories.