Drop the unreachable Instagram branch from the mautrix-meta-messenger image tag

The image tag prefixed `ig-` when `..._meta_mode` was `instagram`, but this role
does not accept that mode any more: `..._database_suffix` and
`..._bridge_username_prefix` map only `facebook`, `facebook-tor` and `messenger`,
so `instagram` raises an undefined-key error long before the tag is rendered.

Instagram has been a separate bridge since v26.07 and is handled by
matrix-bridge-mautrix-meta-instagram, which hardcodes the prefix. Left over from
before that split.

The comment stays, reworded: both bridges still share a container image
repository, so an `ig-` prefix on a tag here would mean this role had pulled the
other bridge's image - which is what the Molecule scenario asserts against.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SEH3vxYSQ5SV4N5z61eyGT

roles/custom/matrix-bridge-mautrix-meta-messenger/defaults/main.yml

@@ -32,12 +32,14 @@ matrix_bridge_mautrix_meta_messenger_container_image: "{{ matrix_bridge_mautrix_
 matrix_bridge_mautrix_meta_messenger_container_image_registry_prefix: "{{ 'localhost/' if matrix_bridge_mautrix_meta_messenger_container_image_self_build else matrix_bridge_mautrix_meta_messenger_container_image_registry_prefix_upstream }}"
 matrix_bridge_mautrix_meta_messenger_container_image_registry_prefix_upstream: "{{ matrix_bridge_mautrix_meta_messenger_container_image_registry_prefix_upstream_default }}"
 matrix_bridge_mautrix_meta_messenger_container_image_registry_prefix_upstream_default: "dock.mau.dev/"
-# Since v26.07, Instagram is a separate bridge upstream (its own binary and its own container image).
-# Both bridges are published to the same container image repository, with Instagram's tags carrying an `ig-` prefix.
+# Since v26.07, Instagram is a separate bridge upstream (its own binary and its own container image),
+# handled by the matrix-bridge-mautrix-meta-instagram role.
+# Both bridges are published to the same container image repository, with Instagram's tags carrying an `ig-` prefix,
+# so an `ig-` prefix on a tag here would mean this role had pulled the other bridge's image.
 # See: https://mau.fi/blog/2026-07-mautrix-release/#new-instagram-bridge
 #
 # Prebuilt container images for specific commit hashes are tagged with an architecture suffix (e.g. `HASH-amd64`).
-matrix_bridge_mautrix_meta_messenger_container_image_tag: "{{ 'ig-' if matrix_bridge_mautrix_meta_messenger_meta_mode == 'instagram' else '' }}{{ matrix_bridge_mautrix_meta_messenger_version }}{{ ('-' ~ matrix_architecture) if (matrix_bridge_mautrix_meta_messenger_version | length == 40) else '' }}"
+matrix_bridge_mautrix_meta_messenger_container_image_tag: "{{ matrix_bridge_mautrix_meta_messenger_version }}{{ ('-' ~ matrix_architecture) if (matrix_bridge_mautrix_meta_messenger_version | length == 40) else '' }}"
 
 matrix_bridge_mautrix_meta_messenger_container_network: ""