Remove --show-annotation FlaggedApi from flagged-apis-droidstubs

framework-connectivity-flagged-apis-droidstubs is used by
jarjar-rules-generator to avoid jarjaring flagged apis.
But, this usage of --show-annotation FlaggedApi in the flag is not
supported.
So this CL removes --show-annotation FlaggedApi.
As far as framework-connectivity-flagged-apis-droidstubs contains
flagged APIs, it's fine that this droidstubs also contains non-flagged
APIs.

Test: TH
Bug: 312408311
Change-Id: I2f1f5624d98873420c0c36ac6e31386102211045