Update uthash to v2.3.0

Also delete files and folders that are only useful in the original
repository. Updating uthash to this version allows compiling with
CHERI LLVM where the current version of uthash (1.9.8) triggers a
warning that our build infrastructure includes in -Werror by default:
```
../../libglvnd/src/util/winsys_dispatch.c:154:9: error: cast from provenance-free integer type to pointer type will give pointer that can not be dereferenced [-Werror,-Wcheri-capability-misuse]
../../libglvnd/include/lkdhash.h:86:5: note: expanded from macro 'LKDHASH_TEARDOWN'
    LKDHASH_TEARDOWN_2(_lh, _param, cur ## _ht,                   \
    ^
../../libglvnd/include/lkdhash.h:55:9: note: expanded from macro 'LKDHASH_TEARDOWN_2'
        HASH_DEL(_LH(_lockedhash), _cur);                                       \
        ^
../../libglvnd/src/util/uthash/src/uthash.h:271:5: note: expanded from macro 'HASH_DEL'
    HASH_DELETE(hh,head,delptr)
    ^
../../libglvnd/src/util/uthash/src/uthash.h:239:14: note: expanded from macro 'HASH_DELETE'
            ((UT_hash_handle*)((ptrdiff_t)_hd_hh_del->next +                     \
             ^
```

Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>
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