ShellPkg: acpiview: XSDT: Remove redundant ParseAcpi() call

ShellPkg: acpiview: XSDT: Remove redundant ParseAcpi() call

Remove a call to ParseAcpi() responsible for getting the XSDT table
length. This call is not needed because the ACPI table buffer length is
provided as an input argument to the ParseAcpiXsdt() function.

Modify remaining code to use the AcpiTableLength argument of the
ParseAcpiXsdt() function instead of a global static variable.

Signed-off-by: Krzysztof Koch <krzysztof.koch@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Zhichao Gao <zhichao.gao@intel.com>