Remove unnecessary `explicit` qualifiers on constructors.

Change-Id: Id12e392ad50f66a6e2251a68662b7959315dc567
diff --git a/compiler/dex/compiler_ir.h b/compiler/dex/compiler_ir.h
index d28df1d..5203355 100644
--- a/compiler/dex/compiler_ir.h
+++ b/compiler/dex/compiler_ir.h
@@ -129,7 +129,7 @@
    * Union containing the option value of either type.
    */
   union OptionContainer {
-    explicit OptionContainer(const OptionContainer& c, OptionType t) {
+    OptionContainer(const OptionContainer& c, OptionType t) {
       if (t == kString) {
         DCHECK(c.s != nullptr);
         s = strndup(c.s, kOptionStringMaxLength);