lint can no longer properly detect unset Summary/Description

lint can no longer properly detect unset Summary/Description

Since the Summary/Description can now be set in the app's source code, or
in fdroiddata/metadata/<packageName>/<locale>/*.txt, this lint check is
no longer valid.  It is important to check whether these texts are empty,
but it'll require some thinking about how and where to best to that.
`fdroid update` will have access to all that data, but perhaps at that
point it is too late.

Also, the current text prioritization puts Summary/Description in the
.txt/.yml file at the highest priority, overriding every other copy,
including in fdroiddata/metadata/<packageName>/<locale> and in the app's
source code.