cephfs-top: Removes unused `global` statements

cephfs-top: Removes unused `global` statements

Recent flake8 runs were failing with:
```
py3: flake8==7.2.0,mccabe==0.7.0,pip==25.0.1,pycodestyle==2.13.0,pyflakes==3.3.0,setuptools==75.8.0,wheel==0.45.1
py3: commands[0] /home/jenkins-build/build/workspace/ceph-pull-requests/src/tools/cephfs/top> flake8 --ignore=W503 --max-line-length=100 cephfs-top
cephfs-top:344:9: F824 `global fs_list` is unused: name is never assigned in scope
cephfs-top:466:13: F824 `global current_states` is unused: name is never assigned in scope
cephfs-top:872:9: F824 `global metrics_dict` is unused: name is never assigned in scope
cephfs-top:872:9: F824 `global current_states` is unused: name is never assigned in scope
cephfs-top:911:9: F824 `global fs_list` is unused: name is never assigned in scope
cephfs-top:981:9: F824 `global current_states` is unused: name is never assigned in scope
cephfs-top:1126:13: F824 `global current_states` is unused: name is never assigned in scope
py3: exit 1 (0.77 seconds) /home/jenkins-build/build/workspace/ceph-pull-requests/src/tools/cephfs/top> flake8 --ignore=W503 --max-line-length=100 cephfs-top pid=2309605
  py3: FAIL code 1 (8.15=setup[7.38]+cmd[0.77] seconds)
  evaluation failed :( (8.24 seconds)
```

Since these variables are only being referenced and not assigned within
their scopes, the `global` declarations are unnecessary and can be
safely removed. This change:

- Removes all flagged `global` statements
- Fixes the failing flake8 checks in the CI pipeline
- Maintains the original code behavior as variable references still work without the `global` keyword

The `global` keyword is only needed when assigning to global variables
within a function scope, not when simply referencing them.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
(cherry picked from commit 39b262f7a6e1d69dc96255a1df62a68297d3f931)