Remove z-index from the eCommerce plan nudge & fix warning

[justinshreve]

This tiny PR removes an unnecessary z-index rule on the eCommerce plan nudge. It was causing a warning on npm start.

*WARNING: No layer found for `.ecommerce-manage-nudge__close-icon` of `[root, .ecommerce-manage-nudge__close-icon]` in $z-layers map. Property omitted.
         on line 306 of assets/stylesheets/shared/functions/_z-index.scss, in function `map-deep-get`
         from line 315 of assets/stylesheets/shared/functions/_z-index.scss, in function `z-index`
         from line 17 of stdin
*

The rule was included when basing the nudge on previous blocks, but the class was not added to the z-layers variable: https://github.com/Automattic/wp-calypso/blob/master/docs/coding-guidelines/css.md#z-index. The rule doesn't seem necessary so I just removed it.

Testing instructions

• npm start and make sure no WARNING appears. It would show up right after Getting bundles ready, hold on....
• Checkout the nudge on an eCommerce plan and see that the close button displays.

[matticbot]

Test live: https://calypso.live/?branch=fix/ecommerce-nudge-z-index

[timmyc]

LGTM