Erroneous `transition-delay` values in genericons.css

Background: [#43195 in WordPress Core Trac](https://core.trac.wordpress.org/ticket/43195).

Per https://developer.mozilla.org/en-US/docs/Web/CSS/time:
> Although unitless zero is allowed for ``s, it's invalid for ``s.

So we could drop `transition-delay` on these two lines since the default is `0s` anyway:

-moz-transition: color .1s ease-in 0;
-webkit-transition: color .1s ease-in 0;

Should be:

-moz-transition: color .1s ease-in;
-webkit-transition: color .1s ease-in;

Open genericons.css and locate the two vendor-prefixed transition declarations containing `ease-in 0`. Check the CSS timing-value guidance linked in the issue, then remove the invalid unitless delay from both declarations. Done means both lines retain the color transition without an explicit delay.

Written by the indexing model from the issue text.