Remove placeholder option from Media Source Plugin (Input processor)

Problem/Motivation

The RIFT module currently provides two separate plugin systems for handling responsive images:

1. RIFT Media Source Plugin (Input Processor): Handles how media entities are processed as input
2. RIFT Source Plugin (Output Processor): Handles how image URLs are generated for output

Currently, there is a "Placeholder" option available in both plugin types, which creates confusion and bugs.

Steps to reproduce

1. Navigate to RIFT configuration /admin/config/rift-ui/config (RIFT UI should be enabled)
2. In the "RIFT Media Source Plugin (Input processor)" dropdown, select "Placeholder"
3. Configure any view mode and attempt to display media entities using RIFT formatters
4. Observe that images stop displaying entirely

Proposed resolution

Remove the Placeholder option from the RIFT Media Source Plugin and direct use the RIFT Source Plugin placeholder options instead.