ShellPkg Shell: Remove an unused global variable
ShellPkg Shell: Remove an unused global variable

The unused global variable InvalidChars is removed.
It is only used in the function IsValidCommandName which
was removed previously.
https://bugzilla.tianocore.org/show_bug.cgi?id=1066

Cc: Jaben Carsey <2d4ee76b36e91bee3a58378b7538904eb027a616@intel.com>
Cc: Ruiyu Ni <9fa3a6894a36bca79e6a72ad65af5a5702efecd1@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: shenglei <bc08ecbbf1316cc766ca5aae38ff0e194af9f1cf@intel.com>
Reviewed-by: Ruiyu Ni <9fa3a6894a36bca79e6a72ad65af5a5702efecd1@intel.com>

MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2