Issue 3541507: Remove deprecated field_layout support from layout_builder.install

Closes #3541507

This change removes all remaining references to the deprecated field_layout module from layout_builder.install. Previously, Layout Builder attempted to migrate existing field_layout settings into its first section when enabling Layout Builder for a display. Since field_layout is deprecated and being removed, that migration code is obsolete and no longer needed.

Changes included:

• Removed field_layout integration code from layout_builder.install.
• Deleted the obsolete LayoutBuilderFieldLayoutCompatibilityTest, which only validated migration from field_layout and is no longer relevant.

Now Layout Builder simply ensures each display has a default section when enabled, without checking for old field_layout configuration.