Remove release_memory feature

Because `sqlite3_db_release_memory` is available in 3.14.0 (min version
supported by rusqlite)
And:
<http://sqlite.org/c3ref/db_release_memory.html>
> Unlike the sqlite3_release_memory() interface, this interface is in effect even when the SQLITE_ENABLE_MEMORY_MANAGEMENT compile-time option is omitted.

src/inner_connection.rs

@@ -382,7 +382,6 @@ impl InnerConnection {
     }
 
     #[inline]
-    #[cfg(feature = "release_memory")]
     pub fn release_memory(&self) -> Result<()> {
         self.decode_result(unsafe { ffi::sqlite3_db_release_memory(self.db) })
     }

src/lib.rs

@@ -652,7 +652,6 @@ impl Connection {
     ///
     /// This calls [`sqlite3_db_release_memory`](https://www.sqlite.org/c3ref/db_release_memory.html).
     #[inline]
-    #[cfg(feature = "release_memory")]
     pub fn release_memory(&self) -> Result<()> {
         self.db.borrow_mut().release_memory()
     }