Search: Fix the remove month+year filter link

[Viper007Bond]

When filtering by month+year, such as "December 2017", the remove URL for that filter cleared the month and day, not the month and year.

Fixes #8328.

Testing instructions:

1. Enable search beta.
2. Enable Jetpack search filter widget.
3. Enable a month filter using code described here: https://jetpack.com/support/search/customize-search/#basic-facets
4. Search for something on your test site.
5. Click on one of the month filters in the widget.
6. Click the remove filter link for that month.
7. Verify that the year has been removed from the URL.

Proposed changelog entry for your changes:

Fixed the link that acts as a month and year filter removal toggle on the Search widget

[gibrown]

LGTM.