Remove unused ACPI_FREE_BUFFER macro. No functional change.

This macro is no longer used by ACPICA and it is not public.
Also update comments related to the use of ACPI_ALLOCATE_BUFFER and
the use of AcpiOsFree to free the buffer. Lv Zheng.
AcpiExec: Fix a few memory leaks in error paths.

This change fixes a few instances where the utility fails to free
returned buffers from ACPICA calls. Lv Zheng.