Remove search themes from an asset group

Removes search themes from a Performance Max asset group. Removal is permanent: a theme can be added again, but its history is gone. Use when: a theme is bringing the wrong traffic and the user has confirmed which ones to drop, by name. Do not use when: the user wants to "pause" a theme — Google has no paused state for signals, so removal is the only option and it is not reversible in the reporting sense. Say that first. Requires confirm_delete: true. Pass the exact theme texts; anything that is not on the asset group is reported back rather than silently ignored. Removing every theme does not stop the campaign: Performance Max keeps serving from the assets and the landing page. If you are unsure which account, campaign or period the user means, ask them — do not guess ids or dates.