CI: Remove outdated unused appveyor config (#1295)
CI: Remove outdated unused appveyor config (#1295)

Required build scripts have already been removed some time ago.
FFroehlich/AMICI,AMICI-developer/AMICI,AMICI-developer/AMICI,FFroehlich/AMICI,FFroehlich/AMICI,AMICI-developer/AMICI,AMICI-developer/AMICI,FFroehlich/AMICI