[appveyor] remove appveyor.yml

[jgrewe]

we are testing windows builds with gh actions anyway

[coveralls]

coverage: 78.615%. remained the same
when pulling 0ea1010 on jgrewe:no_appveyor
into 5e5ec01 on G-Node:master.

[achilleas-k]

Had to remove appveyor from the required checks on github.

[jgrewe]

@achilleas-k thank you! I looked but did not find the Checkbox to remove the requirement for appveyor