fix: Remove redundant trailing slash from API namespace configuration

Trailing slashes are now expected to be providing in the configuration.
This expectation was put in place to simplify configuration logic here
and in host apps.

This change prevents double trailing slashes, which began occurring in
WP-Android in the following. This resulted in failed API requests, like
image uploads.

wordpress-mobile/WordPress-Android#22035

Author: dcalhoun

src/utils/api-fetch.js

@@ -68,7 +68,7 @@ function apiPathModifierMiddleware( options, next ) {
 		// Insert the API namespace after the first two path segments.
 		options.path = options.path.replace(
 			/^(?<apiPath>\/?(?:[\w.-]+\/){2})/,
-			`$<apiPath>${ siteApiNamespace[ 0 ] }/`
+			`$<apiPath>${ siteApiNamespace[ 0 ] }`
 		);
 	}