Remove incorrect/outdated doc for explicit_http_config (#38090)

Commit Message: Remove incorrect/outdated doc for explicit_http_config
Additional Description: Per #38064, this docstring became incorrect with
#14362 so should be removed.
Risk Level: None, doc-only.
Testing: n/a
Docs Changes: Yes it is.
Release Notes: n/a
Platform Specific Features: n/a

Signed-off-by: Raven Black <ravenblack@dropbox.com>

Mirrored from https://github.com/envoyproxy/envoy @ 8ded2fefbc1f051a5bf4e18950db2e4bd543014a

envoy/extensions/upstreams/http/v3/http_protocol_options.proto

@@ -140,7 +140,6 @@ message HttpProtocolOptions {
     option (validate.required) = true;
 
     // To explicitly configure either HTTP/1 or HTTP/2 (but not both!) use ``explicit_http_config``.
-    // If the ``explicit_http_config`` is empty, HTTP/1.1 is used.
     ExplicitHttpConfig explicit_http_config = 3;
 
     // This allows switching on protocol based on what protocol the downstream