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_

Makefile

@@ -94,12 +94,8 @@ PACKAGE_OS_ARCHES := linux_amd64 linux_armv7 linux_arm64
 # All architectures we build Docker images for (Linux only).
 DOCKER_ARCHES := amd64 arm64 armv7
 
-# All ${OS}_${ARCH} combos we build the desktop dylib for.
-DYLIB_ARCHES := darwin_amd64 darwin_arm64
-
 # Computed variables based on the above.
 CODER_SLIM_BINARIES      := $(addprefix build/coder-slim_$(VERSION)_,$(OS_ARCHES))
-CODER_DYLIBS             := $(foreach os_arch, $(DYLIB_ARCHES), build/coder-vpn_$(VERSION)_$(os_arch).dylib)
 CODER_FAT_BINARIES       := $(addprefix build/coder_$(VERSION)_,$(OS_ARCHES))
 CODER_ALL_BINARIES       := $(CODER_SLIM_BINARIES) $(CODER_FAT_BINARIES)
 CODER_TAR_GZ_ARCHIVES    := $(foreach os_arch, $(ARCHIVE_TAR_GZ), build/coder_$(VERSION)_$(os_arch).tar.gz)
@@ -261,26 +257,6 @@ $(CODER_ALL_BINARIES): go.mod go.sum \
 		fi
 	fi
 
-# This task builds Coder Desktop dylibs
-$(CODER_DYLIBS): go.mod go.sum $(MOST_GO_SRC_FILES)
-	@if [ "$(shell uname)" = "Darwin" ]; then
-		$(get-mode-os-arch-ext)
-		./scripts/build_go.sh \
-			--os "$$os" \
-			--arch "$$arch" \
-			--version "$(VERSION)" \
-			--output "$@" \
-			--dylib
-
-	else
-		echo "ERROR: Can't build dylib on non-Darwin OS" 1>&2
-		exit 1
-	fi
-
-# This task builds both dylibs
-build/coder-dylib: $(CODER_DYLIBS)
-.PHONY: build/coder-dylib
-
 # This task builds all archives. It parses the target name to get the metadata
 # for the build, so it must be specified in this format:
 #     build/coder_${version}_${os}_${arch}.${format}