i18n: Merge similar translation strings - login to comment.

[ramiy]

See the attached patch.

[ramiy]

comment.php:

• ​https://build.trac.wordpress.org/browser/trunk/wp-includes/comment.php?marks=3335#L3341

xmlrpc:

• ​https://build.trac.wordpress.org/browser/trunk/wp-includes/class-wp-xmlrpc-server.php?marks=3846#L3850

rest api:

• ​https://build.trac.wordpress.org/browser/trunk/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php?marks=408#L408
• ​https://build.trac.wordpress.org/browser/trunk/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php?marks=425#L425

In all those place login is required to comment (get_option( 'comment_registration' )) and user not logged in.

The patch merges the two similar translation strings into one string.

[SergeyBiryukov]

In 47095:

I18N: Merge duplicate "you must be logged in to comment" strings.

Props ramiy.
Fixes #49251.