Restore direct infill filament selection

[ratdoux]

Description

Restores the sparse infill material selector to Multimaterial > Filament for Features and removes the newer explicit infill override controls:

• enable_infill_filament_override
• infill_filament_use_base_first_layers
• infill_filament_use_base_last_layers

Sparse infill now uses sparse_infill_filament directly again, including slicer/tool-ordering paths, plate extruder collection, modifier config handling, and mixed-filament tests.

Note: These three options were originally added in FullSpectrum as experimental testing controls, but they have since been deemed unnecessary and are removed by this PR.

描述

恢复“多材料 > 功能所用耗材”中的稀疏填充材料选择项，并移除较新的显式填充耗材覆盖控制项：

• enable_infill_filament_override
• infill_filament_use_base_first_layers
• infill_filament_use_base_last_layers

稀疏填充现在会再次直接使用 sparse_infill_filament，包括切片/工具排序路径、打印板耗材收集、修改器配置处理，以及混合耗材相关测试。

说明：这三个选项最初是在 FullSpectrum 中作为实验性测试控制项加入的，但后续评估认为它们没有实际价值，因此本 PR 将其移除。