A mutating admission controller webhook doesn't remove object fields
when instructed to.

E.g. when the JSON patch

  [
    {"op": "remove", "path": "/spec/containers/0/resources/limits/fpga-arria10"},
    {"op": "add", "path": "/spec/containers/0/resources/limits/fpga-interface-id-524abcf", "value": 1}
  ]

is applied to this pod

apiVersion: v1
kind: Pod
metadata:
  name: test-pod
spec:
  restartPolicy: Never
  containers:
    -
      name: test-pod-container
      image: ubuntu:bionic
      imagePullPolicy: IfNotPresent
      command: [ "ls", "-l", "/" ]
      resources:
        limits:
          fpga-arria10: 1

in order to replace the resource name "fpga-arria10" with something understandable
by the device plugin the resulting pod spec still contains the old field plus
a new one. The resulting pod looks like

apiVersion: v1
kind: Pod
metadata:
  name: test-pod
spec:
  restartPolicy: Never
  containers:
    -
      name: test-pod-container
      image: ubuntu:bionic
      imagePullPolicy: IfNotPresent
      command: [ "ls", "-l", "/" ]
      resources:
        limits:
          fpga-arria10: 1
          fpga-interface-id-524abcf: 1

The patch unmarshals patched JSON into a new empty object instead of
existing one. Otherwise JSON unmarshaling reuses existing maps, keeping
existing entries as specified in the "encoding/json" standard package.