LibCore: Remove Stream::is_{readable,writable}

Next to functions like `is_eof` these were really confusing to use, and
the `read`/`write` functions should fail anyways if a stream is not
readable/writable.

Userland/Libraries/LibTLS/TLSv12.h

@@ -356,9 +356,6 @@ private:
     }
 
 public:
-    virtual bool is_readable() const override { return true; }
-    virtual bool is_writable() const override { return true; }
-
     /// Reads into a buffer, with the maximum size being the size of the buffer.
     /// The amount of bytes read can be smaller than the size of the buffer.
     /// Returns either the bytes that were read, or an errno in the case of