[CI][Misc] Remove the gitleaks binary, add it to .gitignore (#15010)

### What this PR does / why we need it?

This PR removes an incorrectly tracked binary `gitleaks` and adds it to
`.gitignore` to prevent future occurrences.

The binary was merged in PR #14852.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

No runtime change. The binary should be installed per dev env for local
CI only.

- vLLM version: v0.27.1
- vLLM main:
https://github.com/vllm-project/vllm/commit/ba07e4a48fc951300d97eb506217dd530583dea3

Signed-off-by: Sijie Fu <fusijie@huawei.com>