ci: Remove incorrectly hardcoded HOST in mac_native task

The HOST is wrong, because this is a native build, where the HOST is
supposed to be auto-detected.

Author: MarcoFalke

ci/test/00_setup_env_mac_native.sh

@@ -6,7 +6,6 @@
 
 export LC_ALL=C.UTF-8
 
-export HOST=x86_64-apple-darwin
 # Homebrew's [email protected] is marked as externally managed (PEP 668).
 # Therefore, `--break-system-packages` is needed.
 export PIP_PACKAGES="--break-system-packages zmq"