[bridge] Fix removal of "has-permission" admission constraint

[geropl]

Fixes #4272.

Test

1. Start a workspace on this branch
2. execute the following script

cd dev/gpctl
go build -v .
./gpctl clusters get-tls-config
./gpctl clusters --name ws-dev register --hint-govern --tls-path=./wsman-tls --url ws-manager:8080
./gpctl clusters list
./gpctl clusters update admission-constraint --name ws-dev add has-permission="test"
./gpctl clusters list
./gpctl clusters update admission-constraint --name ws-dev remove has-permission="test"
./gpctl clusters list

[geropl]

/werft run

👍 started the job as gitpod-build-gpl-gpctl-remove-admission.2

[meysholdt]

Tried it. Works as advertised. Thank you!