Remove deprecated api-cfn and api-cloudwatch classes

heat::api-cloudwatch and heat::api-cfn are not correctly named, these
classes are deprecated and replaced by respectively heat::api_cloudwatch
and heat::api_cfn since release 3.0.0-rc2. It's time to clean it for
release after 4.2.0.

Change-Id: Ib29741173e971cefde1cff67281bce368365dde2