SecurityPkg/TcgPei: drop PeiReadOnlyVariable from Depex

TcgPei doesn't actually use the PEI-phase read-only variable service, so
drop that from the Depex.

This patch was inspired by commit ab9e11da6651 ("SecurityPkg/Tcg2Pei: drop
PeiReadOnlyVariable from Depex", 2018-03-09).

Cc: Chao Zhang <chao.b.zhang@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Suggested-by: Chao Zhang <chao.b.zhang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>