[ClangImporter] Simplify inferDefaultArgument by removing numParams (#27864)
The only thing this was used for is to test if the total number of
parameters was 1, in which case we did the same thing we did for a
first parameter except in one very contrived case: a method with more
than one parameter whose base name starts with "set" and whose first
parameter is an NSZone. There are zero of these in the macOS or iOS
SDKs, and probably even fewer in third-party code.