feat: auth remove: add -b flag

[psturc]

Closes #710

📑 Description

• updated k8sgpt auth remove command to support -b flag and keep it compatible with the previous functionality, i.e. allow specifying multiple backends
• updated README

✅ Checks

• My pull request adheres to the code style of this project
• My code requires changes to the documentation
• I have updated the documentation as required
• All the tests have passed

ℹ Additional Information

Log from local verification

❯ ./bin/k8sgpt auth remove
Error: required flag(s) "backends" not set
Usage:
k8sgpt auth remove [flags]

Flags:
-b, --backends string   Backend AI providers to remove (separated by a comma)
-h, --help              help for remove

Global Flags:
    --config string        Default config file (/Users/psturc/Library/Application Support/k8sgpt/k8sgpt.yaml)
    --kubeconfig string    Path to a kubeconfig. Only required if out-of-cluster.
    --kubecontext string   Kubernetes context to use. Only required if out-of-cluster.

❯ ./bin/k8sgpt auth remove -b
Error: flag needs an argument: 'b' in -b
Usage:
k8sgpt auth remove [flags]

Flags:
-b, --backends string   Backend AI providers to remove (separated by a comma)
-h, --help              help for remove

Global Flags:
    --config string        Default config file (/Users/psturc/Library/Application Support/k8sgpt/k8sgpt.yaml)
    --kubeconfig string    Path to a kubeconfig. Only required if out-of-cluster.
    --kubecontext string   Kubernetes context to use. Only required if out-of-cluster.

❯ ./bin/k8sgpt auth remove -b ""
Error: backends must be set.
The command to remove AI backend provider(s)

Usage:
k8sgpt auth remove [flags]

Flags:
-b, --backends string   Backend AI providers to remove (separated by a comma)
-h, --help              help for remove

Global Flags:
    --config string        Default config file (/Users/psturc/Library/Application Support/k8sgpt/k8sgpt.yaml)
    --kubeconfig string    Path to a kubeconfig. Only required if out-of-cluster.
    --kubecontext string   Kubernetes context to use. Only required if out-of-cluster.

❯ ./bin/k8sgpt auth remove -b non-existing
Error: non-existing does not exist in configuration file. Please use k8sgpt auth new.

❯ cat /Users/psturc/Library/Application\ Support/k8sgpt/k8sgpt.yaml
ai:
  providers:
      - name: noopai
        model: gpt-3.5-turbo
        password: asdf
        temperature: 0.7
      - name: openai
        model: gpt-3.5-turbo
        password: asdf
        temperature: 0.7
      - name: localai
        model: gpt-3.5-turbo
        temperature: 0.7
  defaultprovider: ""
kubeconfig: ""
kubecontext: ""

❯ ./bin/k8sgpt auth remove -b localai,openai
localai deleted from the AI backend provider list
openai deleted from the AI backend provider list

❯ cat /Users/psturc/Library/Application\ Support/k8sgpt/k8sgpt.yaml
ai:
  providers:
      - name: noopai
        model: gpt-3.5-turbo
        password: asdf
        temperature: 0.7
  defaultprovider: ""
kubeconfig: ""
kubecontext: ""

❯ ./bin/k8sgpt auth remove --backends noopai
noopai deleted from the AI backend provider list

❯ cat /Users/psturc/Library/Application\ Support/k8sgpt/k8sgpt.yaml
ai:
  providers: []
  defaultprovider: ""
kubeconfig: ""
kubecontext: ""

[psturc]

@AlexsJones please let me know if there are more changes required for this PR.