Remove redundant dependence in CMake config

Author: snarang181

flang/docs/CMakeLists.txt

@@ -130,7 +130,6 @@ if (LLVM_ENABLE_SPHINX)
     endif()
     if (${SPHINX_OUTPUT_MAN})
       add_sphinx_target(man flang SOURCE_DIR "${CMAKE_CURRENT_BINARY_DIR}/Source")
-      add_dependencies(docs-flang-man gen-FlangCommandLineReference.rst)
       add_dependencies(docs-flang-man copy-flang-src-docs)
     endif()
   endif()