client/Client: pass std::string_view to Write()

Almost all callers have string literal, and the length is known at
compile time.

src/client/Write.cxx

@@ -27,9 +27,3 @@ Client::Write(const void *data, size_t length) noexcept
 	/* if the client is going to be closed, do nothing */
 	return !IsExpired() && FullyBufferedSocket::Write(data, length);
 }
-
-bool
-Client::Write(const char *data) noexcept
-{
-	return Write(data, strlen(data));
-}