[lldb][NFC] Remove eCustomCompletion mode

It's not used by anyone. Also if something implements its own
completion it could just not call the method instead of having
a parameter that makes the function a no-op.

lldb/source/Commands/CommandCompletions.cpp

@@ -29,7 +29,6 @@ using namespace lldb_private;
 
 CommandCompletions::CommonCompletionElement
     CommandCompletions::g_common_completions[] = {
-        {eCustomCompletion, nullptr},
         {eSourceFileCompletion, CommandCompletions::SourceFiles},
         {eDiskFileCompletion, CommandCompletions::DiskFiles},
         {eDiskDirectoryCompletion, CommandCompletions::DiskDirectories},
@@ -47,9 +46,6 @@ bool CommandCompletions::InvokeCommonCompletionCallbacks(
     CompletionRequest &request, SearchFilter *searcher) {
   bool handled = false;
 
-  if (completion_mask & eCustomCompletion)
-    return false;
-
   for (int i = 0;; i++) {
     if (g_common_completions[i].type == eNoCompletion)
       break;