Inline link aria-label fix

[omarreiss]

Context

• Fixes a bug where an inline link that opens in a new window would render undefined in the aria-label

Summary

This PR can be summarized in the following changelog entry:

• Fixes a bug where an inline link that opens in a new window would render undefined in the aria-label.

Relevant technical choices:

• Omit the aria-label entirely until Gutenberg comes up with a better solution.

Test instructions

This PR can be tested by following these steps:

• create a post
• add a paragraph
• select some text in the paragraph and add an external link
• check "Open in new tab" in the add link UI
• save the post
• see the post preview
• inspect the link source and confirm there is no longer an aria label value.

Quality assurance

• I have tested this code to the best of my abilities

Fixes https://yoast.atlassian.net/browse/P1-71