CI: remove `gapanalysis` stage

The gapanalysis stage was useful when we were migrating from OSS analyzers to Semgrep, to ensure parity.

Now that this work has been completed, and with SAST contractors onboarding, this job is causing more permission issues than it is providing value. Hence its deletion.

The compilation of Java and Scala test files done in this stage is still useful and will be integrated back in this repository as part of a follow-up MR.