Eliminate mi_run_to_main, introduce mi_clean_restart

Since we now have mi_runto_main which is like runto_main, eliminate
mi_run_to_main, in favor of a new MI clean_restart counterpart --
mi_clean_restart -- and mi_runto_main.

This makes MI testcases look a bit more like CLI testcases.

gdb/testsuite/ChangeLog:

	* lib/mi-support.exp (mi_clean_restart): New.
	(mi_run_to_main): Delete.
	All callers adjust to use mi_clean_restart / mi_runto_main.

Change-Id: I34920bab4fea1f23fb752928c2969c1f6ad714b6