CMake: Don't invoke QDoc excessively

The html_docs target does not need to call QDoc a third time as
both its dependencies already do that.

With qmake, html_docs did invoke QDoc if CONFIG did not include
'prepare_docs', but none of the Qt modules used that configuration.
We can manage without it for now.

Fixes: QTBUG-87751
Change-Id: I45d2d06006c566c431ae41d7f3a06d6b809987f2
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

cmake/QtDocsHelpers.cmake

@@ -140,22 +140,8 @@ function(qt_internal_add_docs)
 
     add_dependencies(generate_docs_${target} prepare_docs_${target})
 
-    # generate html
-    set(html_qdocs_args
-        -outputdir "${qdoc_output_dir}"
-        -installdir "${INSTALL_DOCDIR}"
-        "${target_source_dir}/${doc_project}"
-        -indexdir "${index_dir}"
-        "${include_path_args}"
-    )
-
-    add_custom_target(html_docs_${target}
-        DEPENDS ${qdoc_bin}
-        COMMAND ${CMAKE_COMMAND} -E env ${qdoc_env_args}
-        ${qdoc_bin}
-        ${html_qdocs_args}
-    )
-
+    # html docs target
+    add_custom_target(html_docs_${target})
     add_dependencies(html_docs_${target} generate_docs_${target})
 
     # generate .qch