MdePkg BaseLib: API PathRemoveLastItem not handle root paths properly

MdePkg BaseLib: API PathRemoveLastItem not handle root paths properly

https://bugzilla.tianocore.org/show_bug.cgi?id=239

When the input path for API PathRemoveLastItem() is a root path like
'fs0:\', the API will return TRUE (indicating a directory or file was
removed from the path) and modifies the path to 'fs0:'. In fact, there's
no directory or file removed in the above case.

This commit adds additional check to resolve this issue and modifies the
API's description to make it more straightforward.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>