Disable missing decl assistant for namespaced ids

This gets rid off another assistant offering useless solutions

REVIEW: 121205
Revert "MissingDeclarationAssistant: Only show identifier"

Otherwise the assistant will show an empty string as target context
in case the decl is to be added in the local context ("Declare 'foo' as
member of ''")

This reverts commit aa7e0a5da8539965cb2a6b393d93f421d47e4719.