StandaloneMmPkg: Remove StandaloneMmDriverEntryPoint.h

StandaloneMmPkg: Remove StandaloneMmDriverEntryPoint.h

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=3235

The library for this header initially resided in StandaloneMmPkg
but moved to MdePkg and now this file is a duplicate of the header
file in MdePkg.

This change removes the header file from StandaloneMmPkg. More
details regarding the history of the library transitioning from
StandaloneMmPkg to MdePkg are below.

The following commit removed the library from StandaloneMmPkg:
  d6253d2f9a33 ("StandaloneMmPkg: remove redundant
  StandaloneMmDriverEntryPoint driver", 2019-03-11)

The following commits added the library class & instance to MdePkg:
  7df4764e6a81a ("MdePkg: introduce standalone MM entry point
  library class", 2019-01-14)

  5866d4992396 ("MdePkg: introduce standalone MM entry point
  library implementation", 2019-01-14)

Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>