remove from_url feature

[andy128k]

Closes #84

[AnderEnder]

@andy128k :

rg from_url .
./src/error.rs
26:    #[cfg(feature = "from_url")]
29:    #[cfg(feature = "from_url")]
39:            #[cfg(feature = "from_url")]
41:            #[cfg(feature = "from_url")]
54:            #[cfg(feature = "from_url")]
56:            #[cfg(feature = "from_url")]
74:#[cfg(feature = "from_url")]
81:#[cfg(feature = "from_url")]

./src/lib.rs
32://! **Note**: This requires enabling the `from_url` feature.
37://! let channel = Channel::from_url("http://example.com/feed.xml").unwrap();
104:#[cfg(feature = "from_url")]

[andy128k]

@AnderEnder thanks. Those are also removed.