ExternalProject: Drop unnecessary 'git submodule init'

Since commit 79410eeb1f (ExternalProject: Initialize Git submodules
recursively and on update (#16083), 2016-04-26, v3.6.0-rc1~105^2) our
`git submodule update` step uses the `--init` flag.  This makes the
prior `git submodule init` unnecessary.

Modules/ExternalProject.cmake

@@ -1141,15 +1141,6 @@ if(error_code)
   message(FATAL_ERROR \"Failed to checkout tag: '${git_tag}'\")
 endif()
 
-execute_process(
-  COMMAND \"${git_EXECUTABLE}\" \${git_options} submodule init ${git_submodules}
-  WORKING_DIRECTORY \"${work_dir}/${src_name}\"
-  RESULT_VARIABLE error_code
-  )
-if(error_code)
-  message(FATAL_ERROR \"Failed to init submodules in: '${work_dir}/${src_name}'\")
-endif()
-
 execute_process(
   COMMAND \"${git_EXECUTABLE}\" \${git_options} submodule update --recursive --init ${git_submodules}
   WORKING_DIRECTORY \"${work_dir}/${src_name}\"