Call global_logger->log_vprintf_() directly without null-checking
global_logger on every log call. Logger::pre_setup() sets global_logger
before any other component is created in the generated setup() function,
so it is guaranteed to be valid by the time any log function is invoked.
Also removes the __FlashStringHelper* esp_log_vprintf_ overload which
was dead code (only called from esp_log_printf_, never directly).
Add a Python codegen test to verify the ordering invariant, and a
comment on App.pre_setup() documenting the constraint.
