[reverse.iter.conv] Remove unclear explicit comments

I don't know what explicit refers to here, it seems to be more appropriate to remove.
People who disagree with me are also welcome.

Author: hewillk

source/iterators.tex

@@ -3358,7 +3358,7 @@
 
 \indexlibrarymember{base}{reverse_iterator}%
 \begin{itemdecl}
-constexpr Iterator base() const;          // explicit
+constexpr Iterator base() const;
 \end{itemdecl}
 
 \begin{itemdescr}