elm_transit: remove unnecessary image preload invocation

Summary:
If user uses image object with following step, then unnecessary preloading
related line is invocated in elm_image_file_set.

elm_image_file_set(obj, file, NULL)
elm_icon_preload_disabled_set(obj, EINA_TRUE)

The following is the preloading related line triggered by elm_image_file_set.

(#0) _image_preload_internal
(#1) _evas_image_load_async_start
(#2) evas_object_image_preload
(#3) _efl_ui_image_smart_internal_file_set
(#4) _efl_ui_image_efl_file_load
(#5) efl_file_load
(#6) efl_file_simple_load
(#7) elm_image_file_set

Moreover there is a flickering issue caused by the unnecessary preloading.
A test code is attached.

Test Plan:
Use following test code. The flickering issue occurs with ELM_SCALE=10 ELM_ACCEL=gl ./test_transit

Reviewers: Hermet, jsuya

Reviewed By: Hermet, jsuya

Subscribers: cedric, reviewers, committers

Tags: efl

Differential Revision: https://phab.enlightenment.org/D11364