ci: remove dylib build pipeline (#22592)

## Summary

The macOS `.dylib` is only used by Coder Desktop macOS v0.7.2 or older.
v0.7.2 was released in August 2025. v0.8.0 of Coder Desktop macOS, also
released in August 2025, uses a signed Coder slim binary from the
deployment instead.

It's unlikely customers will be using Coder Desktop macOS v0.7.2 and the
next release of Coder simultaneously, so I think we can safely remove
this process, given it slows down CI & release processes.

## Changes

- **Makefile**: Remove `DYLIB_ARCHES`, `CODER_DYLIBS` variables and
`build/coder-dylib` target
- **scripts/build_go.sh**: Remove `--dylib` flag and all dylib-specific
logic (c-shared buildmode, CGO, plist embedding, vpn/dylib entrypoint)
- **scripts/sign_darwin.sh**: Remove dylib-specific comment
- **CI (ci.yaml)**: Remove `build-dylib` job, artifact download/insert
steps, and `build-dylib` dependency from `build` job
- **Release (release.yaml)**: Remove `build-dylib` job, artifact
download/insert steps, and `build-dylib` dependency from `release` job
- **vpn/dylib/**: Delete entire directory (`lib.go` + `info.plist.tmpl`)
- **vpn/router.go, vpn/dns.go**: Clean up comments referencing dylib

The slim and fat binary builds are completely unaffected — the dylib was
an independent build target with its own CI job.

_Generated by mux but reviewed by a human_

scripts/sign_darwin.sh

@@ -9,7 +9,6 @@
 # certificate.
 #
 # For the Coder CLI, the binary_identifier should be "com.coder.cli".
-# For the CoderVPN `.dylib`, the binary_identifier should be "com.coder.Coder-Desktop.VPN.dylib".
 #
 # You can check if a binary is signed by running the following command on a Mac:
 #   codesign -dvv path/to/binary