Drop deprecated --async flag support

[navidshaikh]

Description

Drop deprecated --async flag support in the favor of --no-wait flag

Changes

• Drop --async flag
• Remove the respective test cases
• Add CHANGELONG

Reference

Fixes #1089

[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-client-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/kn/commands/service/create.go	82.6%	84.7%	2.1
pkg/kn/commands/service/update.go	90.9%	92.5%	1.5
pkg/kn/flags/bool.go	90.0%	88.1%	-1.9

[rhuss]

Thanks !

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: navidshaikh, rhuss

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