latency-charter.py: import matplotlib once, not twice (#7096)

`matplotlib` is already imported on line 3. % `pipx run ruff --select=F811 .` ``` test/load-generator/latency-charter.py:10:8: F811 Redefinition of unused `matplotlib` from line 3 ```
letsencrypt · Sep 22, 2023 · 574c5cf · 574c5cf
1 parent 519c199
commit 574c5cf
Showing 1 changed file with 0 additions and 1 deletion.
diff --git a/test/load-generator/latency-charter.py b/test/load-generator/latency-charter.py
@@ -7,7 +7,6 @@
 import datetime
 import json
 import pandas
-import matplotlib
 import argparse
 import os
 matplotlib.style.use('ggplot')