Remove GOOS=darwin GOARCH=amd64 cross-compiling example from build.sh.

It doesn't actually work. You get:
	Building webrtc ...
	~/go-webrtc/third_party/webrtc/src ~/go-webrtc
	ERROR at //build/config/BUILDCONFIG.gn:241:3: Assertion failed.
	  assert(host_os == "mac", "Mac cross-compiles are unsupported.")
	  ^-----
	Mac cross-compiles are unsupported.

(The Tor Browser build applies some patches so that the Mac
cross-compile does work, but currently it doesn't work in this repo.)

build.sh

@@ -5,8 +5,8 @@
 # For a cross compile:
 #   GOOS=linux GOARCH=amd64 ./build.sh
 #   GOOS=linux GOARCH=arm ./build.sh
-#   GOOS=darwin GOARCH=amd64 ./build.sh
 # (For a cross-compile from linux-amd64 to linux-arm, you may need to install the binutils-arm-linux-gnueabihf package.)
+# For macOS (GOOS=darwin GOARCH=amd64), you can currently only do a native compile.
 
 PROJECT_DIR=$(pwd)
 THIRD_PARTY_DIR="$PROJECT_DIR/third_party"