remove appveyor badge.

Author: sekineh

README.md

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 # binary-heap-plus-rs
 
 ![Rust](https://github.com/sekineh/binary-heap-plus-rs/workflows/Rust/badge.svg)
-[![Build status](https://ci.appveyor.com/api/projects/status/oewb6667ul5pl05d?svg=true)](https://ci.appveyor.com/project/sekineh/binary-heap-plus-rs)
 
 Enhancement over Rust's `std::collections::BinaryHeap`.