MdeModulePkg: Remove EmuVariableRuntimeDxe

MdeModulePkg: Remove EmuVariableRuntimeDxe

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1323
Merge EmuVariable and Real variable driver.

The real variable driver has been updated to support emulated
variable NV mode.
This patch removes EmuVariableRuntimeDxe after platforms are
migrated to use the merged variable driver.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>
Tested-by: Julien Grall <julien.grall@arm.com>
Acked-by: Julien Grall <julien.grall@arm.com>