Remove security team from CODEOWNERS (#311)

Removes the entire `# Security` section (4 `@dfinity/security` rules +
comment) from `.github/CODEOWNERS`.

These paths are already owned by `@dfinity/dx` via the top-level `*
@dfinity/dx` rule, so effective ownership is unchanged.

Author: tmu0

.github/CODEOWNERS

@@ -13,11 +13,5 @@ public/references/certificates.cddl @dfinity/interface-spec @dfinity/core-protoc
 public/references/requests.cddl @dfinity/interface-spec @dfinity/core-protocol @dfinity/dx
 public/references/http-gateway.did @dfinity/interface-spec @dfinity/core-protocol @dfinity/dx
 
-# Security — product-security team must approve changes to security best practices
-docs/guides/security/ @dfinity/security @dfinity/dx
-docs/concepts/security.md @dfinity/security @dfinity/dx
-docs/references/message-execution-properties.md @dfinity/security @dfinity/dx
-docs/guides/canister-calls/idempotency.md @dfinity/security @dfinity/dx
-
 # Internet Identity
 docs/guides/authentication/ @dfinity/identity @dfinity/dx