Remove unnecessary `explicit` qualifiers on constructors.

Change-Id: Id12e392ad50f66a6e2251a68662b7959315dc567
diff --git a/cmdline/cmdline_parse_result.h b/cmdline/cmdline_parse_result.h
index 717642f..982f178 100644
--- a/cmdline/cmdline_parse_result.h
+++ b/cmdline/cmdline_parse_result.h
@@ -126,7 +126,7 @@
     : CmdlineResult(kSuccess), value_(value), has_value_(true) {}
   explicit CmdlineParseResult(T&& value)
     : CmdlineResult(kSuccess), value_(std::forward<T>(value)), has_value_(true) {}
-  explicit CmdlineParseResult()
+  CmdlineParseResult()
     : CmdlineResult(kSuccess), value_(), has_value_(false) {}
 
   T value_;