Remove trailing slash from the theme Domain Path header check.

The Domain Path header tag shouldn't have a trailing slash, otherwise core will double up the slash when concatenating the path with the POT file.

[Here's where the Domain Path](https://github.com/WordPress/WordPress/blob/4bff4ff2d6623e500785b70f48f241e6969f072b/wp-includes/class-wp-theme.php#L1057) is accounted for (you can see the default below, which doesn't have the trailing slash).

And here's where that path is [prepended to the POT file name](https://github.com/WordPress/WordPress/blob/4bff4ff2d6623e500785b70f48f241e6969f072b/wp-includes/l10n.php#L647).