Remove coveralls badge

We stopped pushing coveralls builds since 2020.

README.md

@@ -4,7 +4,6 @@
 [![Coverity Code Health](https://img.shields.io/coverity/scan/5450.svg)](https://scan.coverity.com/projects/behdad-harfbuzz)
 [![Codacy Code Health](https://api.codacy.com/project/badge/Grade/f17f1708783c447488bc8dd317150eaa)](https://app.codacy.com/app/behdad/harfbuzz)
 [![Codecov Code Coverage](https://codecov.io/gh/harfbuzz/harfbuzz/branch/main/graph/badge.svg)](https://codecov.io/gh/harfbuzz/harfbuzz)
-[![Coverals Code Coverage](https://img.shields.io/coveralls/harfbuzz/harfbuzz.svg)](https://coveralls.io/r/harfbuzz/harfbuzz)
 [![Packaging status](https://repology.org/badge/tiny-repos/harfbuzz.svg)](https://repology.org/project/harfbuzz/versions)
 [ABI Tracker](http://abi-laboratory.pro/tracker/timeline/harfbuzz/)