TUN-7585: Remove h2mux compression

h2mux is already deprecated and will be eventually removed, in the meantime,
the compression tests cause flaky failures. Removing them and the brotli
code slims down our binaries and dependencies on CGO.

h2mux/h2_compressor.go

@@ -0,0 +1,17 @@
+package h2mux
+
+import (
+	"io"
+)
+
+func CompressionIsSupported() bool {
+	return false
+}
+
+func newDecompressor(src io.Reader) decompressor {
+	return nil
+}
+
+func newCompressor(dst io.Writer, quality, lgwin int) compressor {
+	return nil
+}