Improve error msg when serviceaccount.json is in invalid json format

[plkokanov]

How to categorize this PR?

/area usability ops-productivity
/kind enhancement
/platform gcp

What this PR does / why we need it:
This PR enhances the error which is returned if the gcp cloud provider secret contains a serviceaccount.json field which is in an invalid json format and cannot be unmarshalled.
Without this, if someone tries to create a SecretBinding to such a secret containing an invalid serviceaccount.json the only error that is displayed is:

Error from server: error when creating "secretbinding-cloudprovider.yaml": admission webhook "validator.admission-gcp.extensions.gardener.cloud" denied the request: unexpected end of JSON input

Now the error should be more descriptive pointing to its actual cause:

Error from server: error when creating "secretbinding-cloudprovider.yaml": admission webhook "validator.admission-gcp.extensions.gardener.cloud" denied the request: referenced secret foo/bar is not valid: could not get service account from "serviceaccount.json" field: failed to unmarshal json object: unexpected end of JSON input

Additionally, I noticed that the readServiceAccountSecret function is unused, so I removed it.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

An error text which better indicates the reason for the failure is displayed when a user tries to create a `SecretBinding` resource which references a `Secret` with a `serviceaccount.json` field in invalid json format.

[AndreasBurger]

lgtm

[kon-angelo]

/lgtm