[StableHLO] Use stablehlo submodule (#14094)

Use the stablehlo submodule to provide stablehlo, since we do not need
the 'mhlo' part or mlir-hlo anymore.

Fixes: https://github.com/openxla/iree/issues/12678

---------

Co-authored-by: Scott Todd <scotttodd@google.com>
diff --git a/.gitmodules b/.gitmodules
index 384ac8f..22e0981 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -32,9 +32,6 @@
 [submodule "third_party/cpuinfo"]
 	path = third_party/cpuinfo
 	url = https://github.com/pytorch/cpuinfo.git
-[submodule "third_party/mlir-hlo"]
-	path = third_party/mlir-hlo
-	url = https://github.com/iree-org/iree-mhlo-fork.git
 [submodule "third_party/libyaml"]
 	path = third_party/libyaml
 	url = https://github.com/yaml/libyaml.git
@@ -44,3 +41,6 @@
 [submodule "third_party/musl"]
 	path = third_party/musl
 	url = https://github.com/powderluv/musl.git
+[submodule "third_party/stablehlo"]
+	path = third_party/stablehlo
+	url = https://github.com/iree-org/stablehlo.git