EmbeddedPkg: remove nonexistent FLASH_DEFINITION from package .dsc

EmbeddedPkg: remove nonexistent FLASH_DEFINITION from package .dsc

For whatever reason, EmbeddedPkg.dsc included a FLASH_DEFINITION entry
pointing to a nonexistent EmbeddedPkg.fdf.

This used to be silently ignored, but recent BaseTools changes
5e9256cd7f54 ("BaseTools: Guid.xref contain information from FILE statements in FDF")
now caused builds against EmbeddedPkg.dsc to fail.
So delete the redundant entry.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>