fix: ignore delete pod cache error msg

[Donghui0]

Please answer these questions before submitting a pull request

• Why submit this pull request?

• Bugfix

• New feature provided

• Improve performance

• Backport patches

• Related issues
  request help: error log "failed to delete pod from cache"  #684

---

Bugfix

• Description

• How to fix?
  modify the return value ErrPodNoAssignedIP -> nil.

[codecov-commenter]

Codecov Report

Merging #714 (11f7c00) into master (a01888b) will increase coverage by 67.63%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           master      #714       +/-   ##
============================================
+ Coverage   32.36%   100.00%   +67.63%     
============================================
  Files          65         1       -64     
  Lines        6488         1     -6487     
============================================
- Hits         2100         1     -2099     
+ Misses       4137         0     -4137     
+ Partials      251         0      -251     

Impacted Files	Coverage Δ
pkg/types/pod.go
pkg/metrics/prometheus.go
pkg/api/server.go
pkg/apisix/plugin.go
pkg/kube/translation/apisix_ssl.go
pkg/apisix/apisix.go
pkg/ingress/controller.go
pkg/apisix/route.go
pkg/kube/translation/annotations/iprestriction.go
pkg/apisix/consumer.go
... and 54 more

---

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[tao12345666333]

Thanks!
Please fix the unit test case.

--- FAIL: TestPodCacheBadCases (0.00s)
    pod_test.go:39: 
        	Error Trace:	pod_test.go:39
        	Error:      	Not equal: 
        	            	expected: <nil>(<nil>)
        	            	actual  : *errors.errorString(&errors.errorString{s:"pod not running"})
        	Test:       	TestPodCacheBadCases
        	Messages:   	deleting pod
FAIL
coverage: 87.0% of statements
FAIL	github.com/apache/apisix-ingress-controller/pkg/types	0.005s
?   	github.com/apache/apisix-ingress-controller/pkg/types/apisix/v1	[no test files]
ok  	github.com/apache/apisix-ingress-controller/pkg/version	0.003s	coverage: 100.0% of statements
FAIL

[tao12345666333]

Thanks!

LGTM