Remove code to set the ServletResponse contentType and characterEncoding properties from the "Content-Type" header

[rstoyanchev]

Spring MVC generally works with the "Content-Type" header of the response rather than the contentType and characterEncoding properties of ServletResponse.

This is typically done through the HttpHeaders of ServletServerHttpResponse, and then merged into the ServletResponse headers, but as of #36334, we now use ServletResponseHeadersAdapter to map HttpHeaders opertions directly onto ServletResponse header methods.

As mentioned in #36343, Spring MVC has several places where it propagates the "Content-Type" header to the contentType and characterEncoding properties of ServletResponse. This shouldn't be necessary, but it remains in 7.0.x to avoid side effects. Going forward, we can remove it In the next minor version 7.1.