botan2: small cleanup

The explicit setting of the C++ standard to C++11 was
introduced with botan 2.0.1 and is no longer needed.

Signed-off-by: Markus Theil <[e-mail]>

pkgs/development/libraries/botan/2.0.nix

···
   baseVersion = "2.19";
   revision = "3";
   sha256 = "sha256-2uBH85nFpH8IfbXT2dno8RrkmF0UySjXHaGv+AGALVU=";
-  postPatch = ''
-    sed -e 's@lang_flags "@&--std=c++11 @' -i src/build-data/cc/{gcc,clang}.txt
-  '';
 })