fdo#82165: ODF import: clear all shapes when removing header content

Shapes anchored to the first or last paragraph survive setString("")
so need to be deleted with some ruse.

(regression from b8499fc3dcf474050f026b8d5cd1d9037bbe42b7)

Change-Id: I00a8132583c45d1953c207932cc7f02f3065ae77