Plugin translation import - remove_filter fails to remove import status

[timse201]

In the Import_Plugin_Translations WP-CLI command, the add_filter and remove_filter function for the import status do not match.

Currently, the filter is added using an array callback, but the code attempts to remove it using a string.

It should not cause immediate issues for a file import.

The PR also modernizes array key checks using the null coalescing operator

fixes: https://meta.trac.wordpress.org/ticket/8216