Drop unused variables and lambda captures

Also, add context to connect() expressions where we are or were
capturing "this".

Change-Id: I6e006ba6f83d532478018550d148ee93eca59605
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/texteditor/codeassist/codeassistant.cpp

@@ -248,7 +248,7 @@ void CodeAssistantPrivate::requestProposal(AssistReason reason,
     }
     case IAssistProvider::Asynchronous: {
         processor->setAsyncCompletionAvailableHandler(
-            [this, processor, reason](IAssistProposal *newProposal){
+            [this, reason](IAssistProposal *newProposal){
                 QTC_CHECK(newProposal);
                 invalidateCurrentRequestData();
                 displayProposal(newProposal, reason);