Cancel and forget timer in pre-command-hook

Rapid switching between buffers (maybe combined with keyboard-quit),
some using indent-guide-mode and others not, could lead to leftover
timers that indent-guide is not aware of, and to timers acting in
buffers in which indent-guide-mode is disabled.

A spurious timer itself can lead to
- missing overlays in buffers using i-g-m
- overlays in buffers where i-g-m is disabled (with terrible effects,
  since they are not removed before executing the next command)