Remove unused header file from Ruby source code (#17871)

It seems that `ruby/version.h` isn't used now. This PR removes it.

Closes #17871

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/17871 from y-yagi:remove_unused_header be6a4070a6
PiperOrigin-RevId: 670604841

ruby/ext/google/protobuf_c/defs.c

@@ -7,7 +7,6 @@
 
 #include <ctype.h>
 #include <errno.h>
-#include <ruby/version.h>
 
 #include "convert.h"
 #include "message.h"

ruby/ext/google/protobuf_c/protobuf.c

@@ -7,8 +7,6 @@
 
 #include "protobuf.h"
 
-#include <ruby/version.h>
-
 #include "defs.h"
 #include "map.h"
 #include "message.h"