ext/objspace/lib/objspace/trace.rb: Remove the original Kernel#p

... to disable a "method redefined" warning.

http://rubyci.s3.amazonaws.com/solaris11-gcc/ruby-master/log/20210514T050008Z.fail.html.gz
```
  1) Failure:
TestObjSpace#test_objspace_trace [/export/home/chkbuild/chkbuild-gcc/tmp/build/20210514T050008Z/ruby/test/objspace/test_objspace.rb:621]:
<["objspace/trace is enabled"]> expected but was
<["/export/home/chkbuild/chkbuild-gcc/tmp/build/20210514T050008Z/ruby/.ext/common/objspace/trace.rb:29: warning: method redefined; discarding old p",
 "objspace/trace is enabled"]>.
```

ext/objspace/lib/objspace/trace.rb

@@ -26,6 +26,7 @@
 require 'objspace.so'
 
 module Kernel
+  remove_method :p
   define_method(:p) do |*objs|
     objs.each do |obj|
       file = ObjectSpace.allocation_sourcefile(obj)