libsigcxx: removed patch that forced -std=c++11 in pkg-config file

This commit removes a patch from the derivation for libsigc++-2.0 that
appended -std=c++11 to the pkg-config file.

The patch in question orginated from the Debian project to enforce
-std=c++11 in projects using libsigc++-2.0 via pkg-config. This patch
was removed in 2016 when Debian switched to GCC 6.

This fixes NixOS/nixpkgs#33534.