remove cx_oracle from testing

cx_oracle is no longer able to build from its .tar.gz form
reliably because it does not include setuptools in its build
dependencies.   It still can be built if pip is given
--no-build-isolation, or if a wheel file is installed rather than
the .tar.gz, but given how quickly cx_oracle has been pushed
aside by oracledb it's not really that important to be testing
it anymore.

Change-Id: I8a4b2cffabe5275a0df88b5a624ecd6379d84d37

noxfile.py

@@ -49,7 +49,7 @@ DB_CLI_NAMES = {
         "greenlet": {"asyncmy", "aiomysql"},
     },
     "oracle": {
-        "nogreenlet": {"cx_oracle", "oracledb"},
+        "nogreenlet": {"oracledb"},
         "greenlet": {"oracledb_async"},
     },
     "mssql": {