[misc](library): disable ytdl by default

[cosven]

经过一段时间的使用，发现之前 ytdl 这个东西属过度设计（想太多）。

ytdl 实际上只对 feeluown-ytmusic 有用，并且它的配置也很复杂，不如直接把这坨逻辑移到 ytmusic 中去实现。所以先默认关闭这个功能。

[coveralls]

Pull Request Test Coverage Report for Build 13068289375

Details

• 1 of 2 (50.0%) changed or added relevant lines in 2 files are covered.
• 9 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.05%) to 56.414%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
feeluown/app/app.py	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
feeluown/library/library.py	2	63.93%
feeluown/app/app.py	3	55.85%
feeluown/library/ytdl.py	4	13.75%

Totals
Change from base Build 13067962644:	-0.05%
Covered Lines:	10018
Relevant Lines:	17758

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💛 - Coveralls