Http2SolrClient should not use Apache Http client classes

@elyograg pointed out that Http2SolrClient has Apache Http Client class in use and it shouldn't be needed since Http2SolrClient is Jetty client based.

https://lists.apache.org/thread/vz6gldbdq0gylncpwsoshfvlvns4kqv8

Based on some digging, this was introduced here - https://github.com/apache/solr/commit/8efed0555898f485d435d53a72cb6441507d81e1#diff-db379c74efc9ff28acfd7571b07e16dbcdaa510783f2bee835b4a288927b5b3d

It looks like it inadvertently copied from HttpSolrClient.java and used ContentType.parse(ct).getMimeType() instead of MimeTypes.getContentTypeWithoutCharset(ct).