[MINOR] http: add http_remove_header2() to remove a header value.

[MINOR] http: add http_remove_header2() to remove a header value.

Calling this function after http_find_header2() automatically deletes
the current value of the header, and removes the header itself if the
value is the only one. The context is automatically adjusted for a
next call to http_find_header2() to return the next header. No other
change nor test should be made on the transient context though.