Remove stack monitoring (#27163)

We're a super small team (1 UI developer) at the moment and don't want PRs to x-pack/monitoring code to require review from us. We haven't had issues with this approach until now and see no reason to change the policy just yet.

.github/CODEOWNERS

@@ -4,7 +4,6 @@
 /x-pack/plugins/apm/  @elastic/apm-ui
 /x-pack/plugins/beats_management/  @elastic/beats
 /x-pack/plugins/canvas/  @elastic/kibana-canvas
-/x-pack/plugins/monitoring/ @elastic/stack-monitoring 
 /x-pack/plugins/security/  @elastic/kibana-security
 /x-pack/plugins/spaces/  @elastic/kibana-security
 **/*.scss  @elastic/kibana-design