Revert "implement From<Vec<char>> and From<&'a [char]> for String"

This reverts commit ac73335.

src/libcollections/string.rs

@@ -1902,26 +1902,6 @@ impl Into<Vec<u8>> for String {
     }
 }
 
-#[stable(feature = "stringfromchars", since = "1.12.0")]
-impl<'a> From<&'a [char]> for String {
-    #[inline]
-    fn from(v: &'a [char]) -> String {
-        let mut s = String::with_capacity(v.len());
-        for c in v {
-            s.push(*c);
-        }
-        s
-    }
-}
-
-#[stable(feature = "stringfromchars", since = "1.12.0")]
-impl From<Vec<char>> for String {
-    #[inline]
-    fn from(v: Vec<char>) -> String {
-        String::from(v.as_slice())
-    }
-}
-
 #[stable(feature = "rust1", since = "1.0.0")]
 impl fmt::Write for String {
     #[inline]