MdePkg: Enhance PathRemoveLastItem() to support "FS0:File.txt"

MdePkg: Enhance PathRemoveLastItem() to support "FS0:File.txt"

The original implementation only looks for very last backslash
and removes the string after that.
But when the path is like "FS0:File.txt" which doesn't contain
backslash, the function cannot work well.
The patch enhances the code to look for very last backslash or
colon to support the path which doesn't contain backslash.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Tapan Shah <tapandshah@hpe.com>