Remove do-nothing pkgconfig method from dub

dub dependencies allegedly support method:pkgconfig, but this is meaningless
as it's only ever used when the dub method is explicitly requested.
Possibly cut-and-pasted from elsewhere?

mesonbuild/dependencies/base.py

@@ -947,7 +947,7 @@ class DubDependency(ExternalDependency):
 
     @staticmethod
     def get_methods():
-        return [DependencyMethods.PKGCONFIG, DependencyMethods.DUB]
+        return [DependencyMethods.DUB]
 
 class ExternalProgram:
     windows_exts = ('exe', 'msc', 'com', 'bat', 'cmd')