Remove unnecessary external URL building for placeholder image

[AetherUnbound]

Description of Changes

I did an audit of usages of url_for(..., _external=True) in the project. All other uses are for JSON+LD definition of breadcrumb elements which probably explicitly require the full URL with scheme:

OpenOversight/OpenOversight/app/templates/incident_detail.html

Lines 12 to 32 in 6cd9857

	<script type="application/ld+json">
	{
	"@context": "https://schema.org",
	"@type": "BreadcrumbList",
	"itemListElement": [{
	"@type": "ListItem",
	"position": 1,
	"name": "OpenOversight",
	"item": "{{ url_for('main.index', _external=True)|replace('/index','') }}"
	}, {
	"@type": "ListItem",
	"position": 2,
	"name": "{{ incident.department.name|title }} Incidents",
	"item": "{{ url_for('main.incident_api', department_id=incident.department.id, _external=True) }}"
	}, {
	"@type": "ListItem",
	"position": 3,
	"name": "Incident{% if incident.report_number %} {{incident.report_number}}{% endif %}"
	}]
	}
	</script>

All other cases were for email generation which also need the full URL with scheme.

Notes for Deployment

Screenshots (if appropriate)

Tests and linting

• I have rebased my changes on main

• just lint passes

• just test passes