Delete issue templates that don't apply to incubator

### Overview

We need to delete 15 issue templates that were inherited from the Hack for LA website repo and have nothing to do with incubator's AWS infrastructure work, so contributors opening an issue aren't shown 20 choices when only 5 are relevant. The blank issue forms added in #145 now cover general-purpose issues.

### Action Items

- [ ] Delete the 13 website-inherited templates from `.github/ISSUE_TEMPLATE/`:
- [ ] `lighthouse--accessibility---forms.md`
- [ ] `lighthouse--accessibility---links.md`
- [ ] `lighthouse--cross-origin-destinations-are-unsafe.md`
- [ ] `lighthouse--how-to.md`
- [ ] `lighthouse--image-optimization.md`
- [ ] `wave-chrome-extension--accessibility-review.md`
- [ ] `which-accessibility-testing-tool-should-you-use-.md`
- [ ] `add-project-logo-or-image-to-your-main-repository.md`
- [ ] `control-what-appears-when-you-paste-your-sites-link-in-social-media-sites.md`
- [ ] `create-project-card-for--project-name-.md`
- [ ] `update-project-profile---name-of-project-.md`
- [ ] `proposal-to-change-project-md-file-format.md`
- [ ] `update-content-for-readme-file.md`
- [ ] Delete `custom.md`. It is unedited GitHub boilerplate — 126 bytes, empty body, and its `about:` still reads "Describe this issue template's purpose here."
- [ ] Delete `blank-issue.md`. It is superseded by "Blank Issue Form with No Dependency" from #145: same three sections, but as a markdown template it cannot mark fields required and cannot add issues to the project board.
- [ ] Confirm 5 templates remain: the two blank forms, `create-agenda.md`, `update-team-roster.md`, `feature_request.md`.
- [ ] Click New Issue and confirm the picker shows only those five.

### Resources/Instructions

- Directory: [.github/ISSUE_TEMPLATE/](https://github.com/hackforla/incubator/tree/main/.github/ISSUE_TEMPLATE)
- Nothing outside `.github/ISSUE_TEMPLATE/` references any of these files — checked across the repo before writing this issue.
- devops-security did the equivalent cleanup in [devops-security#162](https://github.com/hackforla/devops-security/pull/162); same reasoning applies here.
- Issues already opened from these templates are unaffected — deleting a template does not change existing issues.
- Out of scope, tracked separately: `feature_request.md` is labeled `documentation` and probably wants `enhancement`, and there is no `config.yml`, so the "open a blank issue" escape hatch stays open either way.

Start in `.github/ISSUE_TEMPLATE/` and compare the listed inherited templates, `custom.md`, and `blank-issue.md` with the five templates that should remain. Delete only the specified files, then click New Issue to confirm the picker shows exactly those five templates; existing issues should remain unaffected.

Written by the indexing model from the issue text.