MdeModulePkg/TerminalDxe: Some improvements

MdeModulePkg/TerminalDxe: Some improvements

1. Get default terminal type from PCD rather than using PCANSI
directly in BuildTeminalDevpath;
2. Only terminal type is needed to create an TerminalDev instance, so
remove the useless code of creating and freeing DefaultNode.
3. Some white space refining.

Cc: Feng Tian <feng.tian@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Heyi Guo <heyi.guo@linaro.org>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18027 6f19259b-4bc3-4df7-8a09-765794883524