openmpi: delete all libtool .la files

Clear all *.la from lib to reduce the number file in output.
These libtool files are not needed.

pkgs/development/libraries/openmpi/default.nix

···
   enableParallelBuilding = true;
 
   postInstall = ''
-    rm -f $out/lib/*.la
+    find $out/lib/ -name "*.la" -exec rm -f \{} \;
    '';
 
   postFixup = ''