Remove non-styling IDL CSS classes

[saschanaz]

#1892 removed many CSS classes, but we still have some that dynamically added for test purpose, e.g. idlInterface. Currently they are to make sure we get the correct IDL text, but that purpose can be fulfilled more easily by comparing the whole processed textContent of <pre> with the original untouched one.

What do you think? @marcoscaceres

[marcoscaceres]

Removing them is reasonable so long as we can continue styling things consistently.

[ridhishjain-zepto]

@saschanaz, I got what you've done in #1892, however, the directory structure is changed a lot since then.
can u tell me where to look for the processing part of textContent you've mentioned?