This session type https://searchfox.org/mozilla-central/rev/d6a131ceb435c03ccab2592578f6e2ebf12c1644/devtools/server/actors/watcher/session-context.js#34-40

const SESSION_TYPES = {
  …
  CONTENT_PROCESS: "content-process",

is only used in https://searchfox.org/mozilla-central/rev/d6a131ceb435c03ccab2592578f6e2ebf12c1644/devtools/server/actors/watcher/session-context.js#126-137

/**
 * Create the SessionContext used by the Browser Content Toolbox, to debug only one content process.
 * Or when debugging XpcShell via about:debugging, where we instantiate only one content process target.
 */
function createContentProcessSessionContext() {
  const type = SESSION_TYPES.CONTENT_PROCESS;
  return {
    type,
    supportedTargets: getWatcherSupportedTargets(type),
    supportedResources: getWatcherSupportedResources(type),
  };
}

This was used for the Browser Content Toolbox, which was removed in Bug 1593753, so we can probably cleanup a few more things

Isn't it still used by xpcshell and background mode?
https://searchfox.org/mozilla-central/rev/d6a131ceb435c03ccab2592578f6e2ebf12c1644/devtools/server/actors/descriptors/process.js#99-109