Remove unneeded and/or deprecated styles

[danacotoran]

1. There are numerous instances where we are defining base styles for elements, the downsides of which are highlighted in more detail in the following comments: https://github.com/alphagov/static/pull/2238#discussion_r457199734 https://github.com/alphagov/static/pull/2238#discussion_r457215588 We should no longer be defining styles this way in a componentised GOVUK. In order to remove these styles safely it is necessary to search applications, removing any dependencies of the thing we're aiming to remove from static. Alternatively, if the deprecated styling is only used in one application, we can move them out of static and into that application.

2. There might be many classes in static which are no longer in use, and can therefore be removed https://github.com/alphagov/static/pull/2238#discussion_r457216287 Similar to above, in order to achieve this we should search the applications and either move the classes into the applications that need them, or, if the classes aren't used anywhere, remove them from static entirely