WebSerial.cpp: fixing warnings: `[-Wunused-parameter]`

[MacDada]

They appear when the code is compiled with a flag Wextra.

I know of 3 possible solutions:

• A.) static_cast<void>(unusedParameter); – the one I chose
• B.) (void) unusedParameter; – "old" C–style casting, but A is more precise in C++
• C.) remove unusedParameter var from the function arguments – but keeping the name better explains what it is