wcs_register_plugin_styles() is incorrect

[Jon007]

wcs_register_plugin_styles refers to:
woo-sell-coupons/woo-sell-coupons.css

but the plugin and directory would now normally be wcs-sell-coupons so the css may not be found

[Jon007]

the tiny amount of css is not worth the overhead of an adding an additional file to the site payload, I just commented it out in my version.
Since it may not work currently, it should be ok to remove..

[MarieComet]

Hi @Jon007, after a long time without update, I'm coming back to this repo.
I will look at your issues and PR soon.

[sebastienserre]

Corrected on #21