Fix readline history truncation when length is reduced
The `readline.set_history_length()` function did not previously
truncate the in-memory history when the new length was set to
a value smaller than the current number of history items. This
could lead to unexpected behavior where `get_history_length()`
would still report the old length and writing the history to a
file would write more entries than the new limit.
This patch modifies `set_history_length()` to explicitly
remove the oldest history entries using `remove_history()`
when the length is decreased, ensuring the in-memory history
is correctly truncated to the new limit. This brings the
function's behavior in line with expectations and fixes
failures in `test_write_read_limited_history`.
