BaseTools/PatchCheck.py: Update to handle the two [] as prefix

BaseTools/PatchCheck.py: Update to handle the two [] as prefix

The bug is that only remove the first [] when it does the char count,
however sometimes we use [edk2][patch] as prefix, this patch fix this bug.

Fixes: https://bugzilla.tianocore.org/show_bug.cgi?id=113
Cc: Liming Gao <liming.gao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>