Delete .github/workflows/semgrep.yml (#1296)

## What was changed
Delete the Semgrep workflow file in this repo.

## Why?
Now that the Semgrep CI workflow is shared and runs on every repo, this
per-repo file is no longer needed.

## Checklist

### How was this tested:

Saw the org-level action show up in the Actions tab in other repos and
it runs successfully.

Author: jackdawm

.github/CODEOWNERS

@@ -3,9 +3,3 @@
 # @temporalio/sdk will be requested for review when
 # someone opens a pull request.
 * @temporalio/sdk
-
-# Security team must approve exceptions to static analysis
-# scans, and any custom rules.
-
-.semgrepignore @temporalio/security
-.semgrep/ @temporalio/security