chore: remove legacy top-level Server.h

The Server base class is provided in namespace arduino
by cores/arduino/api/Server.h, which the core's ArduinoAPI.h
already uses. The top-level copy was unused by the core.
cores/arduino/api/deprecated ensure to incluide the correct one.

Signed-off-by: Frederic Pillon <frederic.pillon@st.com>

Author: fpistm