Unlined? Unlined what?

The phrase means her "immobile bronze face" had no wrinkles.

Shall I compare thee to a Summer's day?
Thou art more lovely and more temperate:
Rough winds do shake the darling buds of May,
And Summer's lease hath all too short a date:
Sometime too hot the eye of heaven shines,
And oft' is his gold complexion dimm'd;
And every fair from fair sometime declines,
By chance or nature's changing course untrimm'd:
But thy eternal Summer shall not fade
Nor lose possession of that fair thou owest;
Nor shall Death brag thou wanderest in his shade,
When in eternal lines to time thou growest:

So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee.