Unused imports and dead local variable across graphing modules

[ivankrylov]

Reported by pyflakes, all harmless but worth clearing:

• src/graphing/heatmap.py:200 — import pandas as pd unused
• src/graphing/heatmap.py:370 — local max_pause_ms assigned, never used
• src/graphing/plotting.py:8 — from os import times unused
• src/graphing/plotting.py:9 — re-imports plt, already imported at line 6
• src/graphing/summary.py:5 — from os import times unused
• src/graphing/summary.py:6 — import pandas as pd unused

The duplicated from os import times in two files suggests a copy-paste that was
never intentional.