Ladybird: Remove unused spawn helper

The spawn_helper_process method was introduced together with
get_paths_for_helper_process but was only ever used briefly to spawn
WebContent. Other helper processes (SqlServer, headless_browser etc)
are either execed or spawned with their own helpers & custom arguments.

Ladybird/HelperProcess.h

@@ -12,5 +12,4 @@
 #include <AK/StringView.h>
 #include <LibCore/System.h>
 
-ErrorOr<void> spawn_helper_process(StringView process_name, ReadonlySpan<StringView> arguments, Core::System::SearchInPath, Optional<ReadonlySpan<StringView>> environment = {});
 ErrorOr<Vector<String>> get_paths_for_helper_process(StringView process_name);