Remove the Donate button from the website-wide header

[ghwood]

The button adds complexity and isn't (effectively) used by site visitors, even those looking to support the IETF. (>0.1% of visitors to the home page, and ~2.2% of visitors to /support-us use this). It looks especially redundant in the new website main menu layout. (see screenshot).

The other elements of the header should remain unchanged.