msvc: Remove pthread headers

We use defines to use the equivalent win32 APIs and do not use pthread
on win32, so there is no need to ship these headers.
4 jobs for msvc_cleanup in 2 minutes and 28 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #355745
amd64 debian
build-debian

00:00:37

passed #355746
macos
build-macos

00:00:55

passed #355748
win32 amd64 debian
build-win32

00:01:47

passed #355747
win64 amd64 debian
build-win64

00:01:51