Remove confluent-kafka 1.8 tests

The following is an unresolved issue occuring in the setup of confluent-kafka 1.8.2:
asweigart/PyGetWindow#9

tox.ini

@@ -150,7 +150,7 @@ envlist =
     rabbitmq-messagebroker_pika-{py27,py37,py38,py39,pypy,pypy37}-pika0.13,
     rabbitmq-messagebroker_pika-{py37,py38,py39,py310,pypy37}-pikalatest,
     kafka-messagebroker_confluentkafka-{py27,py37,py38,py39,py310}-confluentkafkalatest,
-    kafka-messagebroker_confluentkafka-{py27,py39}-confluentkafka{0108,0107,0106},
+    kafka-messagebroker_confluentkafka-{py27,py39}-confluentkafka{0107,0106},
     kafka-messagebroker_kafkapython-{pypy,py27,py37,py38,pypy37}-kafkapythonlatest,
     kafka-messagebroker_kafkapython-{py27,py38}-kafkapython{020001,020000,0104},
     python-template_mako-{py27,py37,py38,py39,py310}
@@ -358,7 +358,6 @@ deps =
     messagebroker_pika: tornado<5
     messagebroker_pika-{py27,pypy}: enum34
     messagebroker_confluentkafka-confluentkafkalatest: confluent-kafka
-    messagebroker_confluentkafka-confluentkafka0108: confluent-kafka<1.9
     messagebroker_confluentkafka-confluentkafka0107: confluent-kafka<1.8
     messagebroker_confluentkafka-confluentkafka0106: confluent-kafka<1.7
     messagebroker_kafkapython-kafkapythonlatest: kafka-python