All users of global proxy are gone (*), proxy can be made fully main/cql_test_env local.
(*) one test case still needs it, but can get it via cql_test_env
Closes#13616
* github.com:scylladb/scylladb:
code: Remove global proxy
schema_change_test: Use proxy from cql_test_env
test: Carry proxy reference on cql_test_env