linenoise.cpp: allow completions from empty input

Fixes #1742.

src/linenoise/linenoise.cpp

@@ -2587,13 +2587,6 @@ int InputBuffer::getInputLine(PromptBase& pi) {
 
     // ctrl-I/tab, command completion, needs to be before switch statement
     if (c == ctrlChar('I') && completionCallback) {
-      if (pos == 0)  // SERVER-4967 -- in earlier versions, you could paste
-                     // previous output
-        continue;    //  back into the shell ... this output may have leading
-                     //  tabs.
-      // This hack (i.e. what the old code did) prevents command completion
-      //  on an empty line but lets users paste text with leading tabs.
-
       killRing.lastAction = KillRing::actionOther;
       historyRecallMostRecent = false;