libcamera: buffer: Drop private function setRequest()

There is no need to have a private helper function to access a private
data member when a friend statement is needed anyhow. Remove the helper
function to simplify the code and make it clear that a private member of
Buffer is accessed.

Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

include/libcamera/buffer.h

@@ -129,8 +129,6 @@ private:
 
 	void cancel();
 
-	void setRequest(Request *request) { request_ = request; }
-
 	unsigned int index_;
 	std::array<int, 3> dmabuf_;
 	BufferMemory *mem_;