Upstream: simplified Content-Encoding handling.

Upstream: simplified Content-Encoding handling.

Since introduction of offset handling in ngx_http_upstream_copy_header_line()
in revision 573:58475592100c, the ngx_http_upstream_copy_content_encoding()
function is no longer needed, as its behaviour is exactly equivalent to
ngx_http_upstream_copy_header_line() with appropriate offset.  As such,
the ngx_http_upstream_copy_content_encoding() function was removed.

Further, the u->headers_in.content_encoding field is not used anywhere,
so it was removed as well.

Further, Content-Encoding handling no longer depends on NGX_HTTP_GZIP,
as it can be used even without any gzip handling compiled in (for example,
in the charset filter).