fix: unintentional payload object mappings for checks and notification endpoints

[jstirnaman]

• Removes unnecessary allOf.
• Redundant allOf in discriminators causes some processors (Redoc and Redocly CLI) to create unintended implicit schema mappings.
• For example, for PostCheck In Redoc, this generates the following possible request objects:
  • deadman
  • threshold
  • custom and, the unintended implicit objects:
  • Check
  • PostCheck

Redoc before (current prod):

Redoc after:

This error from @redocly/cli split alerted me and helped me pinpoint the problem:

/usr/local/lib/node_modules/@redocly/cli/lib/commands/split/index.js:167
    const discriminatorPropSchema = obj.properties[obj.discriminator.propertyName];
                                                  ^

TypeError: Cannot read properties of undefined (reading 'type')
    at implicitlyReferenceDiscriminator (/usr/local/lib/node_modules/@redocly/cli/lib/commands/split/index.js:167:51)

[jstirnaman]

@glinton @sanderson