Remove variable store in get_the_excerpt

[niallkennedy]

It looks like get_the_excerpt() either returns a password protected message or post->post_excerpt passed through a filter.

Storing excerpt in a variable should not be necessary; pass directly instead.

[niallkennedy]

return string without first storing it in a variable

[markjaquith]

In [21159]:

Do not store post_excerpt in a temporary variable when we can just return it directly. props niallkennedy. fixes #21041