gdb: get rid of get_displaced_stepping_state

gdb: get rid of get_displaced_stepping_state

Remove function get_displaced_stepping_state.  When it was introduced,
inferiors' displaced stepping state was kept in a linked list in
infrun.c, so it was handy.  Nowadays, the state is kept inside struct
inferior directly, so we can just access it directly instead.

gdb/ChangeLog:

	* infrun.c (get_displaced_stepping_state): Remove, change
	callers to access the field directly.

Change-Id: I9a733e32e29c7ebf856ab0befe1076bbb8c7af69