[SQUASH] Revert "ArmPkg: SmmVariablePei: Introduce variable read serv…

…ice in PEI" (#314)

## Description

This reverts commit 852a5e5, because
the variable PEI module is upstreamed to MdeModulePkg in edk2.

For details on how to complete these options and their meaning refer to
[CONTRIBUTING.md](https://github.com/microsoft/mu/blob/HEAD/CONTRIBUTING.md).

- [x] Impacts functionality?
- [ ] Impacts security?
- [x] Breaking change?
- [ ] Includes tests?
- [ ] Includes documentation?
- [ ] Backport to release branch?

## How This Was Tested

This was tested on QEMU SBSA platform.

## Integration Instructions

Replace the ` ArmPkg/Drivers/SmmVariablePei/SmmVariablePei.inf` with
`MdeModulePkg/Universal/Variable/MmVariablePei/MmVariablePei.inf` in
platform build tree.

ArmPkg/ArmPkg.dsc

@@ -183,7 +183,6 @@
   # MU_CHANGE [BEGIN]
   ArmPkg/Drivers/StandaloneMmCpu/StandaloneMmCpu.inf
   ArmPkg/Library/StandaloneMmCoreEntryPoint/StandaloneMmCoreEntryPoint.inf
-  ArmPkg/Drivers/SmmVariablePei/SmmVariablePei.inf
   # MU_CHANGE [END]
   ArmPkg/Drivers/MmCommunicationPei/MmCommunicationPei.inf