Remove unneeded feature gate

We can use the `parse::hex_remove_optional_prefix` now without the
feature gate.

units/src/locktime/absolute.rs

@@ -6,9 +6,7 @@ use core::fmt;
 
 use internals::error::InputString;
 
-use crate::parse::ParseIntError;
-#[cfg(feature = "alloc")]
-use crate::parse;
+use crate::parse::{self, ParseIntError};
 
 /// The Threshold for deciding whether a lock time value is a height or a time (see [Bitcoin Core]).
 ///