Remove CMake configuration for Sphinx targets in MLIR

MLIR does not have a Sphinx configuration, this is just leading to build
failures at the moment.
The website https://mlir.llvm.org/ is using the Hugo generator to
process the markdown files.

Author: Mehdi Amini

mlir/docs/CMakeLists.txt

@@ -90,14 +90,3 @@ if (LLVM_ENABLE_DOXYGEN)
 endif()
 endif()
 
-if (LLVM_ENABLE_SPHINX)
-  include(AddSphinxTarget)
-  if (SPHINX_FOUND)
-    if (${SPHINX_OUTPUT_HTML})
-      add_sphinx_target(html mlir)
-    endif()
-    if (${SPHINX_OUTPUT_MAN})
-      add_sphinx_target(man mlir)
-    endif()
-  endif()
-endif()