Unfortunately this is not possible due to the way WordPress resolves its URLs. It uses the ‘product-category’ (or any other text for that matter) base of an URL to detect that it is an URL leading to a product category.
When I say it is not possible, that is not completely true. There are SEO plugins that allow you to remove this base, but that can lead to a number of problems with performance and duplicate URLs. You will make it harder for WordPress to detect what page you are trying to reach when you type in a product category URL.
Also understand that the standard ‘Page’ in WordPress always has no base text in the URL. For example:
/about-page/ (this is the URL of a standard page)
/product-category/category-x/ (this is the URL leading to a product category)
What would happen if we remove that ‘product-category’ part?
WordPress will have to do much more work to detect what page you are looking for when entering one of the above URLs. That is why I do not recommend using any SEO plugin to achieve this.
If you still want to do this, look at the WordPress SEO plugin which I believe is capable of doing it: http://wordpress.org/extend/plugins/wordpress-seo/