x86/dmar: zap DMAR signature for dom0 once in TBOOT case
Commit 6c298ecc1f ("vtd: Reinstate ACPI DMAR on system shutdown or
S3/S4/S5") did everything for acpi_dmar_zap() call to be unnecessary,
except for invoking the function from acpi_parse_dmar(), which
123c779379 ("VTd/dmar: Tweak how the DMAR table is clobbered")
added several years later.
Some stale comments are also removed, No functional change.