gdb/varobj: Only re-evaluate invalid globals during re_set

gdb/varobj: Only re-evaluate invalid globals during re_set

When doing varobj_re_set, we currently try to recreate floating varobj.
This was introduced by 4e969b4f0128 "Re-evaluate floating varobj as part
of varobj_invalidate" to deal with use a after free issue.  However
since bc20e562ec0 "Fix use after free in varobj" we now ensure that we
never have dangling pointers so this all recreation is not strictly
needed anymore for floating varobjs.

This commit proposes to remove this recreation process for floating
varobjs.

Tested on x86_64-linux.