slides: only override \sectionpage

slides/sections/0.md

@@ -57,8 +57,7 @@ header-includes: |
   \setbeamerfont{frametitle}{series=\mdseries}
 
   % workaround issue matze/mtheme#371
-  \AtBeginSection{
-    \frame[plain,c,noframenumbering]{
+  \def\sectionpage{
     \centering
     \begin{minipage}{22em}
       \raggedright
@@ -70,7 +69,6 @@ header-includes: |
     \par
     \vspace{\baselineskip}
   }
-  }
 
   \setbeamercovered{transparent}