RFR(T): 8249875: GCC 10 warnings -Wtype-limits with JFR code

[Yasumasa Suenaga]

Hi all,

Please review this issue:

   JBS: https://bugs.openjdk.java.net/browse/JDK-8249875
   webrev: http://cr.openjdk.java.net/~ysuenaga/JDK-8249875/webrev.00/

I saw some warnings in JFR code when I tried to build OpenJDK with GCC 10.1.1 on Fedora 32:

```
/home/ysuenaga/OpenJDK/jdk/src/hotspot/share/jfr/utilities/jfrHashtable.hpp:86:14: warning: comparison of unsigned expression in '>= 0' is always true [-Wtype-limits]
    86 | assert(h >= 0 && h < _table_size, "Illegal hash value");
       | ~~^~~~
```

```
/home/ysuenaga/OpenJDK/jdk/src/hotspot/share/jfr/utilities/jfrDoublyLinkedList.hpp:138:17: warning: comparison of unsigned expression in '>= 0' is always true [-Wtype-limits]
   138 | assert(_count >= 0, "invariant");
```

Both `h` and `_cound` are unsigned type, so we can remove them.
I think this is trivial change.


Thanks,

Yasumasa