fix: remove wrong Host header assignment

[sunfkny]

Mihomo was previously assigning request.Host from the client-provided Host header:

This could cause requests to fail when the client sets an unexpected Host (e.g., jawah/niquests#355). Removing this assignment ensures the Host is derived from the absolute-form URL in readRequest

[sunfkny]

RFC 9112 3.2.2:

When a proxy receives a request with an absolute-form of request-target, the proxy MUST ignore the received Host header field (if any) and instead replace it with the host information of the request-target. A proxy that forwards such a request MUST generate a new Host field value based on the received request-target rather than forward the received Host field value.