net: lib: http_server: Update HTTP2-specific structs/enums naming

For HTTP2-specific structures and enums, use "http2_" prefix to clearly
indicate the distinction from the generic HTTP stuff.

Additionally, some structures/enums describing HTTP2 protocol details
had "server" in the name, while in reality they describe nothing
server-specific. Hence, drop the "server" part where applicable.

Remove unused macros.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>