swift234

[Diagnostics] Remove all categories

Diagnostic categories are entirely unused and arguably useless as
implemented, as they merely denote the sub-component of the
compiler.

As far as categorizing warnings are concerned, I'm abandoning the
effort for now, as the utility is marginal and Swift and the Swift
compiler are probalby not ready for these to be nailed down. For the
sake of cleanliness, the CATEGORY field is also stripped from
WARNINGS.

If there's a need for automatic identifying of compiler sub-components
for diagnstics in the future, there are better ways to do this.

NFC